What is the average menstruation cycle length?

Menstruation, also known as a period or monthly, is the regular discharge of blood and mucosal tissue (known as menses) from the inner lining of the uterus through the vagina. The first period usually begins between twelve and fifteen years of age, a point in time known as menarche. However, periods may occasionally start as young as eight years old and still be considered normal. The average age of the first period is generally later in the developing world, and earlier in the developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days). Bleeding usually lasts around 2 to 7 days. Menstruation stops occurring after menopause, which usually occurs between 45 and 55 years of age. Periods also stop during pregnancy and typically do not resume during the initial months of breastfeeding.
Up to 80% of women report having some symptoms prior to menstruation. Common signs and symptoms include acne, tender breasts, bloating, feeling tired, irritability, and mood changes. These may interfere with normal life, therefore qualifying as premenstrual syndrome, in 20 to 30% of women. In 3 to 8%, symptoms are severe.
A lack of periods, known as amenorrhea, is when periods do not occur by age 15 or have not occurred in 90 days. Other problems with the menstrual cycle include painful periods and abnormal bleeding such as bleeding between periods or heavy bleeding. Menstruation in other animals occur in primates (apes and monkeys).
The menstrual cycle occurs due to the rise and fall of hormones. This cycle results in the thickening of the lining of the uterus, and the growth of an egg, (which is required for pregnancy). The egg is released from an ovary around day fourteen in the cycle; the thickened lining of the uterus provides nutrients to an embryo after implantation. If pregnancy does not occur, the lining is released in what is known as menstruation.

Menstruation

The first menstrual period occurs after the onset of pubertal growth, and is called menarche. The average age of menarche is 12 to 15. However, it may start as early as eight. The average age of the first period is generally later in the developing world, and earlier in the developed world. The average age of menarche has changed little in the United States since the 1950s.

Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).

Perimenopause is when fertility in a female declines, and menstruation occurs less regularly in the years leading up to the final menstrual period, when a female stops menstruating completely and is no longer fertile. The medical definition of menopause is one year without a period and typically occurs between 45 and 55 in Western countries.

During pregnancy and for some time after childbirth, menstruation does not occur. The average length of postpartum amenorrhoea is longer when breastfeeding; this is termed lactational amenorrhoea.

In most women, various physical changes are brought about by fluctuations in hormone levels during the menstrual cycle. This includes muscle contractions of the uterus (menstrual cramping) that can precede or accompany menstruation. Some may notice water retention, changes in sex drive, fatigue, breast tenderness, or nausea. Breast swelling and discomfort may be caused by water retention during menstruation. Usually, such sensations are mild, and some females notice very few physical changes associated with menstruation. A healthy diet, reduced consumption of salt, caffeine and alcohol, and regular exercise may be effective for women in controlling some symptoms. Severe symptoms that disrupt daily activities and functioning may be diagnosed as premenstrual dysphoric disorder. Symptoms before menstruation are known as premenstrual molimina.

Many women experience painful cramps, also known as dysmenorrhea, during menstruation. Pain results from ischemia and muscle contractions. Spiral arteries in the secretory endometrium constrict, resulting in ischemia to the secretory endometrium. This allows the uterine lining to slough off. The myometrium contracts spasmodically in order to push the menstrual fluid through the cervix and out of the vagina. The contractions are mediated by a release of prostaglandins.

Painful menstrual cramps that result from an excess of prostaglandin release are referred to as primary dysmenorrhea. Primary dysmenorrhea usually begins within a year or two of menarche, typically with the onset of ovulatory cycles. Treatments that target the mechanism of pain include non-steroidal anti-inflammatory drugs (NSAIDs) and hormonal contraceptives. NSAIDs inhibit prostaglandin production. With long-term treatment, hormonal birth control reduces the amount of uterine fluid/tissue expelled from the uterus. Thus resulting in shorter, less painful menstruation. These drugs are typically more effective than treatments that do not target the source of the pain (e.g. acetaminophen). Risk factors for primary dysmenorrhea include: early age at menarche, long or heavy menstrual periods, smoking, and a family history of dysmenorrhea. Regular physical activity may limit the severity of uterine cramps.

For many women, primary dysmenorrhea gradually subsides in late second generation. Pregnancy has also been demonstrated to lessen the severity of dysmenorrhea, when menstruation resumes. However, dysmenorrhea can continue until menopause. 5–15% of women with dysmenorrhea experience symptoms severe enough to interfere with daily activities.

Secondary dysmenorrhea is the diagnosis given when menstruation pain is a secondary cause to another disorder. Conditions causing secondary dysmenorrhea include endometriosis, uterine fibroids, and uterine adenomyosis. Rarely, congenital malformations, intrauterine devices, certain cancers, and pelvic infections cause secondary dysmenorrhea. Symptoms include pain spreading to hips, lower back and thighs, nausea, and frequent diarrhea or constipation. If the pain occurs between menstrual periods, lasts longer than the first few days of the period, or is not adequately relieved by the use of non-steroidal anti-inflammatory drugs (NSAIDs) or hormonal contraceptives, women should be evaluated for secondary causes of dysmenorrhea.

When severe pelvic pain and bleeding suddenly occur or worsen during a cycle, the woman or girl should be evaluated for ectopic pregnancy and spontaneous abortion. This evaluation begins with a pregnancy test and should be done as soon as unusual pain begins, because ectopic pregnancies can be life‑threatening.

In some cases, stronger physical and emotional or psychological sensations may interfere with normal activities, and include menstrual pain (dysmenorrhea), migraine headaches, and depression. Dysmenorrhea, or severe uterine pain, is particularly common for young females (one study found that 67.2% of girls aged 13–19 have it).

The medications in the non-steroidal anti-inflammatory drugs (NSAIDs) class are commonly used to relieve menstrual cramps. Some herbs are also claimed to help.

Some women experience emotional disturbances starting one or two weeks before their period, and stopping soon after the period has started. Symptoms may include mental tension, irritability, mood swings, and crying spells. Problems with concentration and memory may occur. There may also be depression or anxiety.

This is part of premenstrual syndrome (PMS) and is estimated to occur in 20 to 30% of women. In 3 to 8% it is severe.

More severe symptoms of anxiety or depression may be signs of premenstrual dysphoric disorder (PMDD). Rarely, in individuals who are susceptible, menstruation may be a trigger for menstrual psychosis.

Extreme psychological stress can also result in periods stopping.

The average volume of menstrual fluid during a monthly menstrual period is 35 milliliters (2.4 tablespoons of menstrual fluid) with 10–80 milliliters (1–6 tablespoons of menstrual fluid) considered typical. Menstrual fluid is the correct name for the flow, although many people prefer to refer to it as menstrual blood. Menstrual fluid contains some blood, as well as cervical mucus, vaginal secretions, and endometrial tissue. Menstrual fluid is reddish-brown, a slightly darker color than venous blood.

About half of menstrual fluid is blood. This blood contains sodium, calcium, phosphate, iron, and chloride, the extent of which depends on the woman. As well as blood, the fluid consists of cervical mucus, vaginal secretions, and endometrial tissue. Vaginal fluids in menses mainly contribute water, common electrolytes, organ moieties, and at least 14 proteins, including glycoproteins.

Many mature females notice blood clots during menstruation. These appear as clumps of blood that may look like tissue. If there are questions (for example, was there a miscarriage?), examination under a microscope can confirm if it was endometrial tissue or pregnancy tissue (products of conception) that was shed. Sometimes menstrual clots or shed endometrial tissue is incorrectly thought to indicate an early-term miscarriage of an embryo. An enzyme called plasmin – contained in the endometrium – tends to inhibit the blood from clotting.

The amount of iron lost in menstrual fluid is relatively small for most women. In one study, premenopausal women who exhibited symptoms of iron deficiency were given endoscopies. 86% of them actually had gastrointestinal disease and were at risk of being misdiagnosed simply because they were menstruating. Heavy menstrual bleeding, occurring monthly, can result in anemia.

There is a wide spectrum of differences in how women experience menstruation. There are several ways that someone's menstrual cycle can differ from the norm, any of which should be discussed with a doctor to identify the underlying cause:
There is a movement among gynecologists to discard the terms noted above, which although they are widely used, do not have precise definitions. Many now argue to describe menstruation in simple terminology, including:

Dysfunctional uterine bleeding is a hormonally caused bleeding abnormality. Dysfunctional uterine bleeding typically occurs in premenopausal women who do not ovulate normally (i.e. are anovulatory). All these bleeding abnormalities need medical attention; they may indicate hormone imbalances, uterine fibroids, or other problems. As pregnant women may bleed, a pregnancy test forms part of the evaluation of abnormal bleeding.

Women who had undergone female genital mutilation (particularly type III- infibulation) a practice common in parts of Africa, may experience menstrual problems, such as slow and painful menstruation, that is caused by the near-complete sealing off of the vagina.

Premature or delayed menarche should be investigated if menarche begins before 9 years, if menarche has not begun by age 15, if there is no breast development by age 13, or if there is no period by 3 years after the onset of breast development.

Sexual intercourse during menstruation does not cause damage in and of itself, but the woman's body is more vulnerable during this time. Vaginal pH is higher and thus less acidic than normal, the cervix is lower in its position, the cervical opening is more dilated, and the uterine endometrial lining is absent, thus allowing organisms direct access to the bloodstream through the numerous blood vessels that nourish the uterus. All these conditions increase the chance of infection during menstruation.

Hormonal contraception affects the frequency, duration, severity, volume, and regularity of menstruation and menstrual symptoms.

The most common form of hormonal contraception is the combined birth control pill, which contains both estrogen and progestogen. It is typically taken in 28-day cycles, 21 hormonal pills with either a 7-day break from pills, or 7 placebo pills during which the woman menstruates. Although the primary function of the pill is to prevent pregnancy, it may be used to treat some menstrual symptoms and syndromes which affect menstruation, such as polycystic ovary syndrome (PCOS), endometriosis, adenomyosis, amenorrhea, menstrual cramps, menstrual migraines, menorrhagia (excessive menstrual bleeding), menstruation-related or fibroid-related anemia and dysmenorrhea (painful menstruation). Using the combined birth control pill, it is also possible for a woman to delay or completely eliminate menstrual periods, a practice called menstrual suppression. Some women do this simply for convenience in the short-term, while others prefer to eliminate periods altogether when possible. This can be done either by skipping the placebo pills, or using an extended cycle combined oral contraceptive pill, which were first marketed in the U.S. in the early 2000s.

Injections such as depo-provera became available in the 1960s. When using progestogen implants, menstruation may be reduced to 3 or 4 menstrual periods per year.

Progestogen-only contraceptive pills (sometimes called the'mini pill') are taken continuously without a 7-day span of using placebo pills, and therefore menstrual periods are less likely to occur than with the combined pill with placebo pills. However, disturbance of the menstrual cycle is common with the mini-pill; 1/3-1/2 of women taking it will experience prolonged periods, and up to 70% experience break-through bleeding (metrorrhagia). Irregular and prolonged bleeding is the most common reason that women discontinue using the mini pill.

When the first birth control pill was being developed, the researchers were aware that they could use the contraceptive to space menstrual periods up to 90 days apart, but they settled on a 28-day cycle that would mimic a natural menstrual cycle and produce monthly periods. The intention behind this decision was the hope of the inventor, John Rock, to win approval for his invention from the Roman Catholic Church. That attempt failed, but the 28-day cycle remained the standard when the pill became available to the public.

There is debate among medical researchers about the potential long-term impacts of these practices upon female health. Some researchers point to the fact that historically, females have had far fewer menstrual periods throughout their lifetimes, a result of shorter life expectancies, as well as a greater length of time spent pregnant or breast-feeding, which reduced the number of periods experienced by females. These researchers believe that the higher number of menstrual periods by females in modern societies may have a negative impact upon their health. On the other hand, some researchers believe there is a greater potential for negative impacts from exposing females perhaps unnecessarily to regular low doses of synthetic hormones over their reproductive years.

Breastfeeding causes negative feedback to occur on pulse secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). Depending on the strength of the negative feedback, breastfeeding women may experience complete suppression of follicular development, follicular development but no ovulation, or normal menstrual cycles may resume. Suppression of ovulation is more likely when suckling occurs more frequently. The production of prolactin in response to suckling is important to maintaining lactational amenorrhea. On average, women who are fully breastfeeding whose infants suckle frequently experience a return of menstruation at fourteen and a half months postpartum. There is a wide range of response among individual breastfeeding women, however, with some experiencing return of menstruation at two months and others remaining amenorrheic for up to 42 months postpartum.

Menstruation is managed by menstruating women to avoid damage to clothing or to accord with norms of public life. Menstrual management practices range from medical suppression of menstruation, through wearing special garments or other items, washing or avoidance of washing, disposal and laundry of stained materials, to separation of menstruators to particular places or activities.

Menstrual products (also called "feminine hygiene" products) are made to absorb or catch menstrual blood. A number of different products are available - some are disposable, some are reusable. Where women can afford it, items used to absorb or catch menses are usually commercially manufactured products.

There are disposable products:

Reusable products include:

In developing countries, many poor women cannot afford commercial feminine hygiene products. Instead, they use materials found in the environment or other improvised materials. Absorption materials that may be used by women who cannot afford anything else include: sand, ash, small hole in earth, cloth, whole leaf, leaf fibre (such as water hyacinth, banana, papyrus, cotton fibre), paper (toilet paper, re-used newspaper, pulped and dried paper), animal pelt e.g. goat skin, double layer of underwear, skirt or sari.

Many religions have menstruation-related traditions, for example: Islam prohibits sexual contact with women during menstruation in the 2nd chapter of the Quran. Some scholars argue that menstruating women are in a state in which they are unable to maintain wudhu, and are therefore prohibited from touching the Arabic version of the Qur'an. Other biological and involuntary functions such as vomiting, bleeding, sexual intercourse, and going to the bathroom also invalidate one's wudhu. In Judaism, a woman during menstruation is called Niddah and may be banned from certain actions. For example, the Jewish Torah prohibits sexual intercourse with a menstruating woman. In Hinduism, menstruating women are traditionally considered ritually impure and given rules to follow. During menstruation, women are not allowed to “enter the kitchen and temples", wear flowers, have sex, touch other males or females.” Western civilization, which has been predominantly Christian, has a history of menstrual taboos.

Menstruation education is frequently taught in combination with sex education at school in Western countries, although girls may prefer their mothers to be the primary source of information about menstruation and puberty. Information about menstruation is often shared among friends and peers, which may promote a more positive outlook on puberty. The quality of menstrual education in a society determines the accuracy of people's understanding of the process. In many Western countries where menstruation is a taboo subject, girls tend to conceal the fact that they may be menstruating and struggle to ensure that they give no sign of menstruation. Effective educational programs are essential to providing children and adolescents with clear and accurate information about menstruation. Schools can be an appropriate place for menstrual education to take place. Programs led by peers or third-party agencies are another option.

Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.

Some anthropologists have questioned the energy cost of rebuilding the endometrium every fertility cycle. It has proposed that the energy savings of not having to continuously maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation).

Many have questioned the evolution of overt menstruation in humans and related species, speculating on what advantage there could be to losing blood associated with dismantling the endometrium, rather than absorbing it, as most mammals do. Humans do, in fact, reabsorb about two-thirds of the endometrium each cycle. Some work asserts that overt menstruation occurs not because it is beneficial in itself. Rather, the fetal development of these species requires a more developed endometrium, one which is too thick to reabsorb completely. There is a correlation between species that have overt menstruation to those that have a large uterus relative to the adult female body size.

Recent reviews suggest that menstruation itself is not an evolved, adaptive trait. Rather, it is an inherent consequence of spontaneous decidualization evolving as a derived trait from non-spontaneous decidualization.

The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking.

Beginning in 1971, some research suggested that menstrual cycles of cohabiting human females became synchronized (menstrual synchrony). However, there is currently significant dispute as to whether menstrual synchrony exists. A 2013 review concluded that menstrual synchrony likely does not exist.

Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles. The first day of menstrual bleeding is the date used for the last menstrual period (LMP). The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, and 21 to 31 days in adults (an average of 28 days).

Menstruation

The first menstrual period occurs after the onset of pubertal growth, and is called menarche. The average age of menarche is 12 to 15. However, it may start as early as eight. The average age of the first period is generally later in the developing world, and earlier in the developed world. The average age of menarche has changed little in the United States since the 1950s.

Perimenopause is when fertility in a female declines, and menstruation occurs less regularly in the years leading up to the final menstrual period, when a female stops menstruating completely and is no longer fertile. The medical definition of menopause is one year without a period and typically occurs between 45 and 55 in Western countries.

During pregnancy and for some time after childbirth, menstruation does not occur. The average length of postpartum amenorrhoea is longer when breastfeeding; this is termed lactational amenorrhoea.

Many women experience painful cramps, also known as dysmenorrhea, during menstruation. Pain results from ischemia and muscle contractions. Spiral arteries in the secretory endometrium constrict, resulting in ischemia to the secretory endometrium. This allows the uterine lining to slough off. The myometrium contracts spasmodically in order to push the menstrual fluid through the cervix and out of the vagina. The contractions are mediated by a release of prostaglandins.

For many women, primary dysmenorrhea gradually subsides in late second generation. Pregnancy has also been demonstrated to lessen the severity of dysmenorrhea, when menstruation resumes. However, dysmenorrhea can continue until menopause. 5–15% of women with dysmenorrhea experience symptoms severe enough to interfere with daily activities.

When severe pelvic pain and bleeding suddenly occur or worsen during a cycle, the woman or girl should be evaluated for ectopic pregnancy and spontaneous abortion. This evaluation begins with a pregnancy test and should be done as soon as unusual pain begins, because ectopic pregnancies can be life‑threatening.

In some cases, stronger physical and emotional or psychological sensations may interfere with normal activities, and include menstrual pain (dysmenorrhea), migraine headaches, and depression. Dysmenorrhea, or severe uterine pain, is particularly common for young females (one study found that 67.2% of girls aged 13–19 have it).

The medications in the non-steroidal anti-inflammatory drugs (NSAIDs) class are commonly used to relieve menstrual cramps. Some herbs are also claimed to help.

Some women experience emotional disturbances starting one or two weeks before their period, and stopping soon after the period has started. Symptoms may include mental tension, irritability, mood swings, and crying spells. Problems with concentration and memory may occur. There may also be depression or anxiety.

This is part of premenstrual syndrome (PMS) and is estimated to occur in 20 to 30% of women. In 3 to 8% it is severe.

More severe symptoms of anxiety or depression may be signs of premenstrual dysphoric disorder (PMDD). Rarely, in individuals who are susceptible, menstruation may be a trigger for menstrual psychosis.

Extreme psychological stress can also result in periods stopping.

The average volume of menstrual fluid during a monthly menstrual period is 35 milliliters (2.4 tablespoons of menstrual fluid) with 10–80 milliliters (1–6 tablespoons of menstrual fluid) considered typical. Menstrual fluid is the correct name for the flow, although many people prefer to refer to it as menstrual blood. Menstrual fluid contains some blood, as well as cervical mucus, vaginal secretions, and endometrial tissue. Menstrual fluid is reddish-brown, a slightly darker color than venous blood.

About half of menstrual fluid is blood. This blood contains sodium, calcium, phosphate, iron, and chloride, the extent of which depends on the woman. As well as blood, the fluid consists of cervical mucus, vaginal secretions, and endometrial tissue. Vaginal fluids in menses mainly contribute water, common electrolytes, organ moieties, and at least 14 proteins, including glycoproteins.

Many mature females notice blood clots during menstruation. These appear as clumps of blood that may look like tissue. If there are questions (for example, was there a miscarriage?), examination under a microscope can confirm if it was endometrial tissue or pregnancy tissue (products of conception) that was shed. Sometimes menstrual clots or shed endometrial tissue is incorrectly thought to indicate an early-term miscarriage of an embryo. An enzyme called plasmin – contained in the endometrium – tends to inhibit the blood from clotting.

The amount of iron lost in menstrual fluid is relatively small for most women. In one study, premenopausal women who exhibited symptoms of iron deficiency were given endoscopies. 86% of them actually had gastrointestinal disease and were at risk of being misdiagnosed simply because they were menstruating. Heavy menstrual bleeding, occurring monthly, can result in anemia.

There is a wide spectrum of differences in how women experience menstruation. There are several ways that someone's menstrual cycle can differ from the norm, any of which should be discussed with a doctor to identify the underlying cause:
There is a movement among gynecologists to discard the terms noted above, which although they are widely used, do not have precise definitions. Many now argue to describe menstruation in simple terminology, including:

Dysfunctional uterine bleeding is a hormonally caused bleeding abnormality. Dysfunctional uterine bleeding typically occurs in premenopausal women who do not ovulate normally (i.e. are anovulatory). All these bleeding abnormalities need medical attention; they may indicate hormone imbalances, uterine fibroids, or other problems. As pregnant women may bleed, a pregnancy test forms part of the evaluation of abnormal bleeding.

Women who had undergone female genital mutilation (particularly type III- infibulation) a practice common in parts of Africa, may experience menstrual problems, such as slow and painful menstruation, that is caused by the near-complete sealing off of the vagina.

Premature or delayed menarche should be investigated if menarche begins before 9 years, if menarche has not begun by age 15, if there is no breast development by age 13, or if there is no period by 3 years after the onset of breast development.

Sexual intercourse during menstruation does not cause damage in and of itself, but the woman's body is more vulnerable during this time. Vaginal pH is higher and thus less acidic than normal, the cervix is lower in its position, the cervical opening is more dilated, and the uterine endometrial lining is absent, thus allowing organisms direct access to the bloodstream through the numerous blood vessels that nourish the uterus. All these conditions increase the chance of infection during menstruation.

Hormonal contraception affects the frequency, duration, severity, volume, and regularity of menstruation and menstrual symptoms.

Injections such as depo-provera became available in the 1960s. When using progestogen implants, menstruation may be reduced to 3 or 4 menstrual periods per year.

Progestogen-only contraceptive pills (sometimes called the'mini pill') are taken continuously without a 7-day span of using placebo pills, and therefore menstrual periods are less likely to occur than with the combined pill with placebo pills. However, disturbance of the menstrual cycle is common with the mini-pill; 1/3-1/2 of women taking it will experience prolonged periods, and up to 70% experience break-through bleeding (metrorrhagia). Irregular and prolonged bleeding is the most common reason that women discontinue using the mini pill.

When the first birth control pill was being developed, the researchers were aware that they could use the contraceptive to space menstrual periods up to 90 days apart, but they settled on a 28-day cycle that would mimic a natural menstrual cycle and produce monthly periods. The intention behind this decision was the hope of the inventor, John Rock, to win approval for his invention from the Roman Catholic Church. That attempt failed, but the 28-day cycle remained the standard when the pill became available to the public.

Menstruation is managed by menstruating women to avoid damage to clothing or to accord with norms of public life. Menstrual management practices range from medical suppression of menstruation, through wearing special garments or other items, washing or avoidance of washing, disposal and laundry of stained materials, to separation of menstruators to particular places or activities.

Menstrual products (also called "feminine hygiene" products) are made to absorb or catch menstrual blood. A number of different products are available - some are disposable, some are reusable. Where women can afford it, items used to absorb or catch menses are usually commercially manufactured products.

There are disposable products:

Reusable products include:

In developing countries, many poor women cannot afford commercial feminine hygiene products. Instead, they use materials found in the environment or other improvised materials. Absorption materials that may be used by women who cannot afford anything else include: sand, ash, small hole in earth, cloth, whole leaf, leaf fibre (such as water hyacinth, banana, papyrus, cotton fibre), paper (toilet paper, re-used newspaper, pulped and dried paper), animal pelt e.g. goat skin, double layer of underwear, skirt or sari.

Most female mammals have an estrous cycle, yet only ten primates species, four bats species, the elephant shrew, and the spiny mouse have a menstrual cycle. The relatedness of these groups suggests that four distinct evolutionary events have caused menstruation to arise.

Some anthropologists have questioned the energy cost of rebuilding the endometrium every fertility cycle. It has proposed that the energy savings of not having to continuously maintain the uterine lining more than offsets energy cost of having to rebuild the lining in the next fertility cycle, even in species such as humans where much of the lining is lost through bleeding (overt menstruation) rather than reabsorbed (covert menstruation).

Recent reviews suggest that menstruation itself is not an evolved, adaptive trait. Rather, it is an inherent consequence of spontaneous decidualization evolving as a derived trait from non-spontaneous decidualization.

The grandmother hypothesis is a hypothesis to explain the existence of menopause in human life history by identifying the adaptive value of extended kin networking.

Beginning in 1971, some research suggested that menstrual cycles of cohabiting human females became synchronized (menstrual synchrony). However, there is currently significant dispute as to whether menstrual synchrony exists. A 2013 review concluded that menstrual synchrony likely does not exist.

Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.

Menstrual synchrony

Menstrual synchrony, also called the McClintock effect, is an alleged process whereby women who begin living together in close proximity experience their menstrual cycle onsets (the onset of menstruation or menses) becoming more synchronized together in time than when previously living apart. "For example, the distribution of onsets of seven female lifeguards was scattered at the beginning of the summer, but after 3 months spent together, the onset of all seven cycles fell within a 4-day period."

Martha McClintock's 1971 paper, published in "Nature", says that menstrual cycle synchronization happens when the menstrual cycle onsets of two or more women become closer together in time than they were several months earlier..

After the initial studies, several papers were published reporting methodological flaws in studies reporting menstrual synchrony including McClintock's study. In addition, other studies were published that failed to find synchrony. The proposed mechanisms have also received scientific criticism. A 2013 review concluded that menstrual synchrony likely does not exist.

Martha McClintock published the first study on menstrual synchrony among women living together in dormitories at Wellesley College, a women's liberal arts college.

McClintock hypothesized that pheromones could cause menstrual cycle synchronization. However, other mechanisms have been proposed, most prominently synchronization with lunar phases.

No scientific evidence supports the lunar hypothesis, and doubt has been cast on pheromone mechanisms.

After the initial studies reporting menstrual synchrony began to appear in the scientific literature, other researchers began reporting the failure to find menstrual synchrony.

These studies were followed by critiques of the methods used in early studies, which argued that biases in the methods used produced menstrual synchrony as an artifact.

More recent studies, which took into account some of these methodological criticisms, failed to find menstrual synchrony.

The term "synchrony" has been argued to be misleading because no study has ever found that menstrual cycles become strictly concordant, nevertheless "menstrual synchrony" is used to refer the phenomenon of menstrual cycle onsets becoming closer to each other over time.

In a recent systematic review of menstrual synchrony, Harris and Vitzthum concluded, "In light of the lack of empirical evidence for MS [menstrual synchrony] "sensu stricto", it seems there should be more widespread doubt than acceptance of this hypothesis" (pp. 238–239).

The experience of synchrony may be the result of the mathematical fact that menstrual cycles of different frequencies repeatedly converge and diverge over time and not due to a process of synchronization. It may also be because of the high probability of menstruation overlap that occurs by chance.

Researchers are divided on whether menstrual synchrony would be adaptive. McClintock has suggested that menstrual synchrony may not be adaptive but rather epiphenomenonal, lacking any biological function. Among those who postulate an adaptive function, one argument is that menstrual synchrony is only a particular aspect of the much more general phenomenon of reproductive synchrony, an occurrence familiar to ecologists studying animal populations in the wild. Whether seasonal, tidal or lunar, reproductive synchrony is a relatively common mechanism through which co-cycling females can increase the number of males included in the local breeding system.

Conversely, it has been argued that if there are too many females cycling together, they would be competing for the highest quality males; forcing female-female competition for high quality mates and thereby lowering fitness. In such cases, selection should favor avoiding synchrony. Divergent climate regimes differentiating Neanderthal reproductive strategies from those of modern "Homo sapiens" have recently been analysed in these terms.

One additional hypothesis, postulated by Van Arsdale and Tyler, is that menstrual synchronization allows for synchronized birthing, therefore enhancing access to vital nutrition in cases of maternal mortality across tribal populations. In other words, synchronized birthing allowed babies who had lost their primary mothers during or after childbirth to have access to breast milk, thereby improving the chances of child survival.

Turning to the evolutionary past, a possible adaptive basis for the biological capacity would be reproductive levelling: among primates, synchronising to any natural clock makes it difficult for an alpha male to monopolise fertile sex with multiple females. This would be consistent with the striking gender egalitarianism of extant non-storage hunter-gatherer societies. A further deep-time evolutionary pressure may have been lions' habit of eating people on moonless nights. When early Pleistocene hominids in Africa were attempting to survive by robbing big cats of their kills, according to some evolutionary scientists, it may have been adaptive to restrict overnight journeys—including sexual liaisons—to times when there was a moon in the sky.

The question of whether those who live together do in fact synchronize their menstrual cycles has also received attention in the popular media.

The idea that menstruation is – or ideally ought to be – in harmony with wider cosmic rhythms is one of the most tenacious ideas central to the myths and rituals of traditional communities across the world.

"String was first made by the two Wawalik Sisters at Mudawa, near Buckingham Bay... The sisters sat down, looking at each other, with their feet out and legs apart, and both menstruated… Each one made a loop of the other one’s menstrual blood, after which they put the string loops around their necks."

The!Kung (or Ju|'hoansi) hunter-gatherers of the Kalahari "believe...that if a woman sees traces of menstrual blood on another woman's leg or even is told that another woman has started her period, she will begin menstruating as well". Among the Yurok people of northwestern California, according to one ethnographic study, "all of a household's fertile women who were not pregnant menstruated at the same time..."

The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women. The phenomenon is not synchronization in the strict sense of concordance of menstrual cycle onsets but the term "menstrual synchrony" is still used perhaps misleadingly. As an undergraduate, Martha McClintock published the first study on menstrual synchrony; her report detailed the menstrual synchrony of undergraduate women living in a dormitory in Wellesley College. Since then, there have been attempts to replicate her findings and to determine the conditions under which synchrony occurs, if it exists. Her work was followed up by studies reporting menstrual synchrony and by other studies that failed to find synchrony.

Thus, a number of studies were published from the 1980s to the mid 2000s, which attempted to replicate menstrual synchrony in college women, determine the conditions under which menstrual synchrony occurred, and to address methodological issues that were raised as these studies were published. The rest of this section discusses these studies in chronological order, briefly presenting their findings and main conclusions grouped by decade followed by general methodological issues in menstrual synchrony research.

McClintock's study consisted of 135 female college students who were 17 to 22 years old at the time of the study. They were all residents of a single dormitory, which had four main corridors. The women were asked when their last and second to last menstrual period had started three times during the academic year (which ranged from September to April). They also were asked who (other women in the dormitory) they associated with most and how often each week they associated with males. From these data, McClintock placed women into pairs of close friends and roommates and she also placed them into groups of friends ranging in size from 5 to 10 women. She reported statistically significant synchrony for both her pairwise sorting of women and her group sorting of women. That is, whether women were placed into pairs of close friends and roommates or whether they were placed into larger groups of friends, she reported that they synchronized their menstrual cycles. She also reported that the more often women associated with males, the shorter their menstrual cycles were. She speculated that this may be a pheromone effect paralleling the Whitten effect in mice but that it could not explain menstrual synchrony among women. Finally, she speculated that there could be a pheromone mechanism of menstrual synchrony similar to the Lee-Boot effect in mice.

Graham and McGrew were the first researchers to attempt to replicate McClintock's study. There were 79 women living in halls of residence or apartments on the campus of a college in Scotland. The women were 17 to 21 years old at the time of the study and the procedures followed were similar to those used in McClintock's study. She partially replicated McClintock's study reporting that close friends but not neighbors synchronized their cycles. Unlike in McClintock's study, close friends did not synchronize in groups. They considered a pheromone mechanism a possible explanation of synchrony, but noted that if pheromones were the cause, neighbors should have synchronized as well. They concluded that the mechanism of synchrony remains unknown, but emotional attachment may play a role.

Quadagno et al. conducted the second replication of McClintock's study. There were 85 women living in dormitories, sorority houses, and apartments who attended a large midwestern university in the United States. Their study used methods similar to McClintock's study except in addition to two women living together, there were also groups of three and four women living together. They reported that the women synchronized their menstrual cycles and concluded that pheromones may have played a role in synchronization.

Jarett's study was the third to attempt to replicate McClintock's original study using college roommates. There were 144 women who attended two colleges. The women were 17 to 22 years old and the procedures followed were similar to McClintock's study except only pairs of roommates were used. She reported that the women did not synchronize. Jarett concluded that whether menstrual synchrony occurs in a group of women may depend on the variability of their menstrual cycles. She conjectured that the reason the women in her study did not synchronize their menstrual cycles was because they happened to have longer and more irregular menstrual cycles than in McClintock's original study.

Wilson, Kiefhabe, and Gravel conducted two studies with college women. The first study consisted of 132 women who were members of a sorority or roommates of members at the University of Missouri. The women were 18 to 22 years of age and the study aimed to replicate McClintock's original study. However, instead of asking women to recall when their last and next to last menstrual onsets occurred, one of the researchers visited the sorority daily to record the occurrence of menstrual onsets and to collect other biographical data. The second study consisted of 24 women living in a cooperative house near the University of Missouri. The women were 18 to 31 years of age. One of the researchers visited the house three times a week recording menstrual onset and collecting more extensive biographical and psychological test data than in the first study. They found no menstrual synchrony in either study. They considered the possibility that women with irregular cycles may reduce the likelihood of detecting synchrony, so they re-analyzed their data after they removed women with irregular cycles, but again there was no statistically significant effect of synchrony. They concluded that "It is clear no meaningful process of selection or exclusion of pairs can produce a significant level of menstrual synchrony in our samples... Therefore, whether or not menstrual synchrony occurs among women who spend time together must remain a hypothesis requiring further investigation" (p. 358).

Weller and Weller conducted a study with 20 lesbian couples. They hypothesized that contact within couples should be maximal and contact with men minimal compared to previous studies, which should maximize the likelihood of detecting synchrony. The women ranged in age between 19 and 34 years of age. This was the first study that did not explicitly use college women, but instead the women were recruited at a bar by a research assistant who was a proprietor of a bar. Unlike previous studies, they only asked the women for the date of their last menstrual onset. They then assumed that all the women had menstrual cycles that were exactly 28 days long. Based on this assumption and one menstrual onset for each woman in a couple, they calculated the degree of synchrony. They reported that more than half of the couples had synchronized within a two-day timespan of each other.

Trevathan, Burleson, and Gregory also conducted a study with 29 lesbian couples (22 to 48 years of age), but they incorporated the methodological critique of Wilson into the design of their study. In particular, Wilson emphasized the importance of using actual menstrual cycle lengths with their inherent variability. The lesbian couples were drawn from a larger sample of women who had kept daily records of their menstrual cycles for three months and who had participated in a previous study. They found no evidence of synchrony. They discussed several factors that could have prevented synchrony in their study but they strongly suggested that menstrual synchrony may not be a real phenomenon because of the methodological issues Wilson raised and because menstrual synchrony appears to lack adaptive significance.

In addition to the study they conducted with lesbian couples, Weller and Weller conducted a number of other studies on menstrual synchrony during the 1990s. In most studies they reported finding menstrual synchrony, but in some studies they did not find synchrony. In a methodological review article in 1997, they refined their approach to measuring to better handle the problem of cycle variability. Specifically, they concluded that several menstrual cycles should be measured from each woman and that the longest average cycle length in a pair or group of women should be the basis for calculating the expected cycle onset difference. Thus, their research falls into the pre-1997 methodology and post-1997 methodology.

In 1997, Weller and Weller published one of the first studies to investigate when menstrual synchrony occurs in complete families. Their study was conducted in Bedouin villages in northern Israel. Twenty seven families, which had from two to seven sisters 13 years or older and collected data on menstrual cycle onsets over a three-month period. Using the methods of, they reported menstrual synchrony occurred for the first two months, but not for the third month for roommate sisters, close friend roommates, and for families as a whole.

Strassmann investigated whether menstrual synchrony occurred in a natural fertility population of Dogon village women. Her study consisted of 122 Dogon women with an average lifetime fertility rate of 8.6 ±.3 live births per woman. Their median cycle length was 30 days, which is indistinguishable from western women. In analyzing whether menstrual synchrony occurs among Dogon women, she was aware of Wilson's methodological criticisms of previous studies and aware that menstrual synchrony isn't synchrony "per se", but rather the closeness of menstruation among women. She used Cox regression to determine whether the likelihood of menstruating was influenced by other women. She considered the levels of all the women in the village, all the women in the same lineage, and all the women in the same economic unit (i.e., they worked together). She found no significant relationship at any level, which means that there was no evidence of synchronization. She concluded that this result undermined the view that menstrual synchrony is adaptive and the view held by many anthropologists at the time that menstrual synchrony occurred in preindustrial societies.

Menstrual synchrony research declined after the published critiques in the 1990s and around the turn of the century. The two studies published during this decade incorporated the methodological critiques into their designs and used more appropriate statistical methods.

Yang and Schank conducted the largest study to date with 186 Chinese college women. Ninety three of the women lived in 13 dorm rooms, 5 to 8 women per room. The other ninety three women lived in 16 dorm rooms, 4 to 8 women per room for a total of 29 rooms. The women were given notebooks to record the onset of each of their cycles and they collected data for over a year for most of the women.

Following the statistical critiques of Schank, they argued that circular statistics were required to analyze periodic data for the existence of synchrony. However, menstrual cycles are variable in frequency (e.g., 28 or 31 day cycles) and in length. They pointed out that there are no statistical methods for analyzing messy data like this, so they developed Monte Carlo methods for detecting synchrony.

They found that in 9 of the 29 groups, women's cycles converged for one cycle closer than expected by chance, but then they diverged again. Upon further analysis, they found that for women with the cycle variability reported in this study, on average 10 out of 29 groups of women would show this pattern of convergence followed by divergence. They concluded that finding 9 out of 29 groups with convergence and then divergence is about what would be expected by chance and concluded that there was no evidence the women in this study synchronized their menstrual cycles.

Ziomkiewicz conducted a study with 99 Polish college women living in two dormitories. Thirty six of the women lived in 18 double rooms and sixty three lived in 21 triple rooms. Women recorded their menstrual cycle onsets on menstrual calendars provided to them and 181 days' worth of menstrual cycle data were collected. The mean menstrual cycle length was 30.5 days (SD = 4.56).

Based on the mean cycle length of the women in this study, the expected difference by chance in menstrual cycle onset was approximately 7.5 days. The mean difference in cycle onset was calculated for the beginning, middle, and end of the study for the pairs and triples of women. Ziomkiewicz found no statistically significant difference from the 7.5 day expected difference at either the beginning, middle, or end of the study. She concluded that there was no evidence of menstrual synchrony.

H. Clyde Wilson argued that at the start of any menstrual synchrony study, the minimum cycle onset difference must be calculated by using two onset differences from each woman in a group. For example, suppose two women have exactly 28-day cycles. The greatest distance that one cycle onset can be from another is 14 days. Suppose the first two onsets for woman A are July 1 and July 29 and for woman B, they are July 24 and August 21. If only the first two recorded onsets of A and B are compared, the difference between onsets is 23 days, which is greater than the 14 days that can actually occur. Wilson argued that McClintock did not correctly calculate the initial onset difference among women and concluded that the initial onset difference among women in a group was biased towards asynchrony.

Yang and Schank followed up on this point by using computer simulations to estimate the average onset difference that would occur by among women with variable cycle lengths and a mean cycle length of 29.5 days reported by McClintock. They reported that the average onset difference by chance among women with cycle characteristics reported by McClintock was about 5 days. They also calculated the expected onset difference at the beginning of the study using McClintock's method for calculating initial cycle onset differences. They reported that the initial cycle onset difference for the groups of women using McClintock's method was about 6.5 days. McClintock reported that groups of women had an initial cycle onset difference at the beginning of her study of about 6.5 days and then subsequently synchronized to an average difference of a little less than 5 days. Yang and Schank point out that since the expected cycle onset differences they calculated were so close to the differences reported by McClintock, that there may be no statistical difference. They concluded that If their analysis is correct, it implies that synchrony did not occur in McClintock's original study.

Cutler and Law hypothesized that menstrual synchrony is caused by menstrual cycles synchronizing with lunar phases. However, neither of them agree on what phase of the lunar cycle menstrual cycles synchronize with. Cutler hypothesizes the synchronization with the full moon and Law with the new moon. Neither offer hypotheses regarding how lunar phases cause menstrual synchrony and neither are consistent with previous studies that found no relationship between menstrual cycles and lunar cycles. More recently, Strassmann investigated menstrual synchrony among Dogon village women. The women were outdoors most nights and did not have electrical lighting. She hypothesized that Dogon women would be ideal for detecting a lunar influence on menstrual cycles, but she found no relationship.

Jarett hypothesized that women who were more affiliative and concerned with how others viewed them would synchronize more. In her study, however, women with low affiliation scores were associated with greater synchrony. She found that women with a need for social recognition and approval from others were associated with synchrony, which is partially consistent with her hypothesis. Nevertheless, the group of women she studied did not synchronize their menstrual cycles.

When McClintock published her study on menstrual synchrony, she speculated that pheromones may cause menstrual synchrony. In a study on Norway rats, McClintock proposed and tested a coupled oscillator hypothesis (see section on rats below). The coupled-oscillator hypothesis proposed estrous cycles in rats were cause by two, estrous phase dependent pheromones that mutually modulated the length of cycles in a group and thereby causing synchrony.

This idea was extended to humans in a study by Stern and McClintock. They investigated whether a coupled-oscillator mechanism first reported for Norway rats (see section below on rats) could also exist in humans. The coupled-oscillator hypothesis in humans proposed that human females release and receive pheromones that regulate the length of their menstrual cycles. This was assumed to occur without consciously detecting any odor. The study was conducted by collecting compounds from axillae (underarms) of donor women at prescribed phases during their menstrual cycles (i.e., the follicular phase, ovulatory phase, and luteal phase), and applying the compounds daily under the noses of recipient women. In order to collect the axillary compounds, the donor women wore cotton pads under their arms for at least 8 hours, and then the pads were cut into smaller squares, frozen to preserve the scent, and readied for distribution to the recipients. The recipients were split into two groups, and were exposed to the compounds via application of the thawed axillary pad under their noses daily.

The researchers concluded that odorless compounds collected from women during the late follicular phase of their menstrual cycles triggered hormonal events that shortened the menstrual cycles of the recipient women, and that odorless compounds collected from women during the time of ovulation triggered a hormonal event in the recipient women that lengthened their menstrual cycles. Stern and McClintock concluded that these findings "proved the existence of human pheromones" as well as illustrated manipulation of the human menstrual cycle.

Researchers pointed out several flaws in their study. Whitten's main critiques was with their using only their first cycles as a control for the subsequent conditions. He argued that this eliminate all within-subject variance. Control conditions should have been run between each experimental condition and just at the beginning of the study. He was also skeptical about whether the coupled-oscillator model from rat research could be applied to humans.

Arden and Dye investigated women's awareness and perception of menstrual synchrony. Their study consisted of 122 women (students and staff) at Leeds University. A four-page questionnaire was sent to each participant. After providing personal details, they were given a description of menstrual synchrony: "Menstrual synchrony occurs when two or more women, who spend time with each other, have their periods at approximately the same time" (p. 257) After reading the description they were asked whether they were aware of menstrual synchrony and whether they had experienced it. They were then asked details about their experience of synchrony such as how many times they experienced and how long it lasted.

They found that 84% of the women were aware of the phenomenon of menstrual synchrony and 70% reported the personal experience of synchrony. The experience of synchrony occurred most commonly with close friends followed by roommates. There was considerable variation in the reported time spent together before synchrony occurred ranging from zero to four weeks to 12 months or more. The most common time was 12 months or more. The duration of menstrual synchrony also was highly variable with responses ranging from one to two months to 12 months or more. They conclude that "Whether or not future research concludes that menstrual synchrony is an objective phenomenon, subjective experiences, which are apparently widespread, need to be given careful consideration." (p. 265)

Both Wilson and Arden and Dye pointed out that menstrual synchrony can occur by chance when there is menstrual cycle variability. Yang and Schank argued that when there is cycle variability (i.e., either women have irregular cycles, have cycles of different frequencies, or both), most women will have the opportunity to experience synchrony even though it is a result of cycle variability and not a result of a mechanism such as the exchange of pheromones. For example, consider two women A and B. Suppose A has menstrual cycles that are 28 days long and B has cycles that are 30 days long. Suppose further that when A and B become close friends, B has a cycle onset 14 days before A's next onset. The next time both of them have menstrual cycle onsets, B will have a cycle onset 12 days before A. B will continue to gain two days on A until their onsets coincide, then their cycles will begin to diverge again. The cycles of A and B will repeatedly converge and diverge creating the appearance of synchrony during convergence. This is a mathematical property of cycles of different frequencies and not due to the interaction of A and B. If, in addition, the duration of menstruation is considered (typically 3 to 5 days with a range of 2 to 7 days), then the experience of synchrony may last a number of months.

Strassmann argued menstrual synchrony defined as menstruation overlap should be quite common. For example, the expected difference by chance between two women with 28-day cycles—which is approximately the average length of menstrual cycles of women at the age —is 7 days. Considering that the mean duration of menses is 5 days and the range is 2 to 7 days, the probability of menstruation overlap by chance should be high.

In order to work out why menstrual synchrony might have evolved, it is necessary to investigate why individuals who synchronized their cycles might have had increased survival and reproduction in the evolutionary past. The relevant field in this case is behavioral ecology.

In mammalian mating systems generally, and among primates in particular, female spatio-temporal distribution – how clumped females are in the environment and how much they overlap their fertile periods – affects the ability of any single male to monopolize matings. The basic principle is that the more females are fertile at any one time, the harder it is for any single male to monopolize access to them, impregnating all simultaneously at the expense of rival males. In the case of nonhuman primates, once the number of co-cycling females rises above a critical threshold, a harem-holder may be unable to prevent other males from invading and mating with his females. A dominant male can maintain his monopoly only if his females stagger their fertile periods, so that he can impregnate them one at a time (see figure a, right). Suppose a group of female baboons need between them just one dominant male, desirable in view of his high-quality genes. Then, logically, they should avoid synchronizing their cycles. By the same token, if males during the course of human evolution became valued by females for additional purposes – hunting and bringing home food, for example – then females should resist being controlled by dominant male harem-holders. If males are useful partners to have and keep around, then ideally each female should have at least one for herself. Under those circumstances, according to this argument, the logical strategy would be for females to synchronize as tightly as they can (see figure b, right).

One implication is that there may be a link between the degree of synchrony in a population (whether seasonal, lunar or both), and the degree of reproductive egalitarianism among males. Foley and Fitzgerald objected to the idea that synchrony could have been a factor in human evolution on the grounds that for hominins with inter-birth intervals of 3–5 years, achieving synchrony was unrealistic. Infant mortality would disrupt synchrony since it would be too costly for a mother who had miscarried or lost her baby to wait until everyone else had weaned their babies and resumed cycling before having sex and getting pregnant herself. On the other hand, while conceding that it would be impossible to get clockwork synchrony throughout an inter-birth interval, Power et al. argued that once we take account of birth seasonality – enhancing the effects of menstrual synchrony by clumping fertile cycles within a relatively brief time-window – it emerges that reproductive synchrony can be effective as a female strategy to undermine primate-style sexual monopolization by dominant males. The controversy remains unresolved.

Adopting a compromise position, one school of Darwinian thought argues that the human female may once have had adaptive reasons for evolving a 29.5-day menstrual cycle – implying some theoretical potential for synchrony to a lunar clock – but did so in an African setting under prehistoric conditions which today no longer exist. Not all archaeologists accept that lunar periodicity was ever relevant to human evolution. On the other hand, according to Curtis Marean (head of excavations at the important Middle Stone Age site of Pinnacle Point, South Africa), anatomically modern humans around 165,000 years ago – when inland regions of the continent were dry, arid and uninhabitable – became restricted to small populations clustered around coastal refugia, reliant on marine resources including shellfish whose safe harvesting at spring low tides presupposed careful tracking of lunar phase.
Estrous synchrony, a phenomenon similar to menstrual synchrony,
has been reported in several other mammalian species.

Menstrual or estrous synchrony has been reported in other species including Norway rats, hamsters, chimpanzees, and Golden lion tamarins. In non-human primates, the term may also refer to the degree of overlap of menstrual or estrous cycles, which is the overlap of estrous or menses of two or more females in a group due, for example, to seasonal breeding.

However, as with early human studies on menstrual synchrony, non-human estrous synchrony studies also were criticized for methodological problems.

Subsequent studies failed to find estrous synchrony in rats, hamsters, chimpanzees, and Golden lion tamarins.

McClintock also conducted a 1978 study of estrous synchrony in Norway rats ("Rattus norvegicus"). She reported that the estrous cycles of female rats living in groups of five were more regular than those of rats housed singly. She also reported that social interaction, and more importantly a shared air supply that allowed for olfactory communication enhanced the regularity of the rats’ cycles and synchronized their estrous phases after two or three cycles. McClintock hypothesized that estrous synchrony was caused by pheromones and that a coupled oscillator mechanism produced estrous synchrony in rats This observation of menstrual synchrony in Norway rats is not the same as the Whitten effect because it was the result of the continuous interactions of ongoing cycles within a female group, rather than the result of an exposure to a single external stimulus such as male odor, which in the Whitten effect releases all exposed females simultaneously from an acyclic condition.

The coupled-ocillator hypothesis asserted that females rats release two pheromone signals. One signal is released during the follicular phase of the estrous cycle and it shortens estrous cycles. The second signal is released during the ovulatory phase of the estrous cycle and it lengthens estrous cycles. When rats live together or share the same air supply, the pheromones released by each female in a group as a function of the phase of her estrous cycle causes other females in the group to either lengthen or shorten their estrous cycles. This mutual lengthening and shortening of estrous cycles was theorized to produce synchronization of estrous cycles over time.

McClintock investigated the coupled oscillator hypothesis experimentally. She provided three groups of rats with airborne odors from female rats in three different phases of the estrous cycle: ovulatory phase, follicular phase, and luteal phase. She hypothesized that ovulatory phase odors would lengthen cycles, follicular phase odors would shorten cycles, and luteal phase odors would have no effect. Her results showed a lengthening of estrous cycles for females who received ovulatory odors, shortening of cycles for females who received follicular odors, and no effect for females who received luteal phase odors.

The coupled-ocillator hypothesis was also investigated using a computer simulation model, which was compared with data from McClintock's 1978 study. They found that a coupled oscillator mechanism could produce estrous synchrony in female rats, but the effect was very weak. The proposed mechanisms of this model were more precisely tested by controlling the airborne odors received by individual females. They found support for the hypothesis that follicular phase odors short the length of estrous cycles, but they did not find that ovulatory phase odors lengthened cycles as the earlier study by McClintock had found.

Schank conducted another experiment to test whether female rats could synchronize their cycles. He found that female rats did not synchronize their cycles and he argued that in the original McClintock study, the random control group was more asynchronous than expected by chance. When the experimental group was compared to the control group in McClintock's 1978 study, the experimental group was more synchronous than the control group but only because the control group was too asynchronous and not because the experimental group had synchronized their cycles. In a follow-up study, Schank again found no effect of estrous synchrony in rats.

In 1980, estrous synchrony was reported in female hamsters. In their study, hamsters were housed in four colony phase of the estrous cycle. They monitored and females in each room and removed the females that did not stay in phase. They placed a wire metal cage (i.e., condo consisting of four equally sized rectangular compartments) in the corner of each room. For each room, three animals were randomly selected and placed in three of the condo compartments. A fourth female was randomly selected from another room and placed in the remaining condo compartment. In the control condition, all four females placed in the condos came from the same room. Females were kept in the condos until all four animals exhibited 4 consecutive days of synchrony. They were then removed and a new group was formed until all combinations were tested. They found that the fourth female in the experimental condition always synchronized with the remaining three

Their study was criticized as methodologically flawed because females were left together until the fourth female synchronized with the others. When female hamsters are subjected to the stress of stranger hamsters, their cycles become irregular. If only the female from another room's cycles change, then by chance, the longer the female is left with the other three, the more likely it is that she will synchronize by chance with the other three. In a follow-up experimental study motivated by this methodological critique, no evidence for estrous synchrony was found for female hamsters.

In 1985, estrous synchrony was reported in female chimpanzees. In her study, 10 female chimpanzees were caged, at different times, in two groups of four and six in the same building. The anogenital swelling of each female was recorded daily. Synchrony was measured by calculating the absolute differences in days between (1) the day of swelling onset and (2) the day of maximum swelling. She reported a statistically significant average difference of 5.7 days for onset of swelling and 8.0 days for maximum swelling. Schank, however, noted that due to females who became pregnant and who stopped cycling, most of the data were based on only four animals. He performed a computer simulation study to calculate the expected swelling onset and maximal swelling onset difference for female chimpanzees with the reported mean estrous cycle lengths of 36.7 (with a standard deviation of 4.3) days. He reported an expected difference of 7.7 days. Thus, a maximum swelling difference of 8.0 days is about what would be expected by chance and given that only four animals contributed data to the study, a 5.7 day onset difference is not significantly less than 7.7 days.

Since then Matsumoto and colleagues have reported estrous asynchrony in groups of free-living chimpanzees in Mahale Mountains National Park, Tanzania. They subsequently investigated whether estrous asynchrony was adaptive for female chimpanzees. They tested three hypotheses about the adaptiveness of estrous asynchrony: (1) females become asynchronous to increase copulation frequency and opportunities for giving birth; (2) paternity confusion to reduce infanticide; and (3) sperm competition. They found no support for hypothesis (1) and partial support for hypotheses (2) and (3).

In 1987, estrous synchrony was reported in female golden lion tamarins by French and Stribley. Their subjects consisted of five adult female golden lion tamarins that were housed in two groups. Two females were housed with adult males and three females (a mother and two daughters) were housed with an adult male and infant male. They reported a 2.11 day difference in peak cycle estrogen for the two groups, which was less than the 4.5 day difference that they calculated would be the difference based on golden lion tamarins having a 19-day estrous cycle. Schank reanalyzed their study with the help of computer simulation and reported that a 2.11 day difference was not likely statistically significant. Monfort and colleagues conducted a study with eight females housed in pairs and found no evidence of synchrony.

Setchella, Kendala, and Tyniec investigated whether menstrual synchrony occurred in a semi-free-ranging population of mandrills of 10-group years. They reported that mandrills do not synchronize their menstrual cycles and concluded that cycle synchrony does not occur in non-human primates.

Oestrus synchrony has been reported of female lions living under natural conditions in the wild.

The menstrual cycle is the regular natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible. The cycle is required for the production of oocytes, and for the preparation of the uterus for pregnancy. Up to 80% of women report having some symptoms during the one to two weeks prior to menstruation. Common symptoms include acne, tender breasts, bloating, feeling tired, irritability and mood changes. These symptoms interfere with normal life and therefore qualify as premenstrual syndrome in 20 to 30% of women. In 3 to 8%, they are severe.
The first period usually begins between twelve and fifteen years of age, a point in time known as menarche. They may occasionally start as early as eight, and this onset may still be normal. The average age of the first period is generally later in the developing world and earlier in developed world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women and 21 to 35 days in adults (an average of 28 days). Menstruation stops occurring after menopause which usually occurs between 45 and 55 years of age. Bleeding usually lasts around 2 to 7 days.
The menstrual cycle is governed by hormonal changes. These changes can be altered by using hormonal birth control to prevent pregnancy. Each cycle can be divided into three phases based on events in the ovary (ovarian cycle) or in the uterus (uterine cycle). The ovarian cycle consists of the follicular phase, ovulation, and luteal phase whereas the uterine cycle is divided into menstruation, proliferative phase, and secretory phase.

Menstrual cycle

Stimulated by gradually increasing amounts of estrogen in the follicular phase, discharges of blood (menses) flow stop, and the lining of the uterus thickens. Follicles in the ovary begin developing under the influence of a complex interplay of hormones, and after several days one or occasionally two become dominant (non-dominant follicles shrink and die). Approximately mid-cycle, 24–36 hours after the luteinizing hormone (LH) surges, the dominant follicle releases an ovocyte, in an event called ovulation. After ovulation, the ovocyte only lives for 24 hours or less without fertilization while the remains of the dominant follicle in the ovary become a corpus luteum; this body has a primary function of producing large amounts of progesterone. Under the influence of progesterone, the uterine lining changes to prepare for potential implantation of an embryo to establish a pregnancy. If implantation does not occur within approximately two weeks, the corpus luteum will involute, causing a sharp drop in levels of both progesterone and estrogen. The hormone drop causes the uterus to shed its lining in a process termed menstruation. Menstruation also occurs in closely related primates (apes and monkeys).

The average age of menarche is 12–15. They may occasionally start as early as eight, and this onset may still be normal. This first period often occurs later in the developing world than the developed world.

The average age of menarche is approximately 12.5 years in the United States, 12.7 in Canada, 12.9 in the UK and 13.1 years in Iceland. Factors such as genetics, diet and overall health can affect timing.

The cessation of menstrual cycles at the end of a woman's reproductive period is termed menopause. The average age of menopause in women is 52 years, with anywhere between 45 and 55 being common. Menopause before age 45 is considered "premature" in industrialised countries. Like the age of menarche, the age of menopause is largely a result of cultural and biological factors; however, illnesses, certain surgeries, or medical treatments may cause menopause to occur earlier than it might have otherwise.

The length of a woman's menstrual cycle typically varies somewhat, with some shorter cycles and some longer cycles. A woman who experiences variations of less than eight days between her longest cycles and shortest cycles is considered to have regular menstrual cycles. It is unusual for a woman to experience cycle length variations of more than four days. Length variation between eight and 20 days is considered as moderately irregular cycles. Variation of 21 days or more between a woman's shortest and longest cycle lengths is considered very irregular.

The average menstrual cycle lasts 28 days. The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39. Subsequently, the variability increases slightly for women aged 40 to 44.

The luteal phase of the menstrual cycle is about the same length in most individuals (mean 14.13 days, standard deviation 1.41 days) whereas the follicular phase tends to show much more variability (log-normally distributed with 95% of individuals having follicular phases between 10.3 and 16.3 days). The follicular phase also seems to get significantly shorter with age (geometric mean 14.2 days in women aged 18–24 vs. 10.4 days in women aged 40–44).

Some women with neurological conditions experience increased activity of their conditions at about the same time during each menstrual cycle. For example, drops in estrogen levels have been known to trigger migraines, especially when the woman who suffers migraines is also taking the birth control pill. Many women with epilepsy have more seizures in a pattern linked to the menstrual cycle; this is called "catamenial epilepsy". Different patterns seem to exist (such as seizures coinciding with the time of menstruation, or coinciding with the time of ovulation), and the frequency with which they occur has not been firmly established. Using one particular definition, one group of scientists found that around one-third of women with intractable partial epilepsy has catamenial epilepsy. An effect of hormones has been proposed, in which progesterone declines and estrogen increases would trigger seizures. Recently, studies have shown that high doses of estrogen can cause or worsen seizures, whereas high doses of progesterone can act like an antiepileptic drug. Studies by medical journals have found that women experiencing menses are 1.68 times more likely to attempt suicide.

Mice have been used as an experimental system to investigate possible mechanisms by which levels of sex steroid hormones might regulate nervous system function. During the part of the mouse estrous cycle when progesterone is highest, the level of nerve-cell GABA receptor subtype delta was high. Since these GABA receptors are inhibitory, nerve cells with more delta receptors are less likely to fire than cells with lower numbers of delta receptors. During the part of the mouse estrous cycle when estrogen levels are higher than progesterone levels, the number of delta receptors decrease, increasing nerve cell activity, in turn increasing anxiety and seizure susceptibility.

Estrogen levels may affect thyroid behavior. For example, during the luteal phase (when estrogen levels are lower), the velocity of blood flow in the thyroid is lower than during the follicular phase (when estrogen levels are higher).

Among women living closely together, it was once thought that the onsets of menstruation tend to synchronize. This effect was first described in 1971, and possibly explained by the action of pheromones in 1998. Subsequent research has called this hypothesis into question.

Research indicates that women have a significantly higher likelihood of anterior cruciate ligament injuries in the pre-ovulatory stage, than post-ovulatory stage.

The most fertile period (the time with the highest likelihood of pregnancy resulting from sexual intercourse) covers the time from some 5 days before until 1 to 2 days after ovulation. In a 28‑day cycle with a 14‑day luteal phase, this corresponds to the second and the beginning of the third week. A variety of methods have been developed to help individual women estimate the relatively fertile and the relatively infertile days in the cycle; these systems are called fertility awareness.

There are many fertility testing methods, including urine test kits that detect the LH surge that occurs 24 to 36 hours before ovulation; these are known as ovulation predictor kits (OPKs). Computerized devices that interpret basal body temperatures, urinary test results, or changes in saliva are called fertility monitors. Fertility awareness methods that rely on cycle length records alone are called calendar-based methods. Methods that require observation of one or more of the three primary fertility signs (basal body temperature, cervical mucus, and cervical position) are known as symptoms-based methods.

A woman's fertility is also affected by her age. As a woman's total egg supply is formed in fetal life, to be ovulated decades later, it has been suggested that this long lifetime may make the chromatin of eggs more vulnerable to division problems, breakage, and mutation than the chromatin of sperm, which are produced continuously during a man's reproductive life. However, despite this hypothesis, a similar paternal age effect has also been observed.

As measured on women undergoing in vitro fertilization, a longer menstrual cycle length is associated with higher pregnancy and delivery rates, even after age adjustment. Delivery rates after IVF have been estimated to be almost doubled for women with a menstrual cycle length of more than 34 days compared with women with a menstrual cycle length shorter than 26 days. A longer menstrual cycle length is also significantly associated with better ovarian response to gonadotropin stimulation and embryo quality.

The different phases of the menstrual cycle correlate with women's moods. In some cases, hormones released during the menstrual cycle can cause behavioral changes in females; mild to severe mood changes can occur. The menstrual cycle phase and ovarian hormones may contribute to increased empathy in women. The natural shift of hormone levels during the different phases of the menstrual cycle has been studied in conjunction with test scores. When completing empathy exercises, women in the follicular stage of their menstrual cycle performed better than women in their midluteal phase. A significant correlation between progesterone levels and the ability to accurately recognize emotion was found. Performances on emotion recognition tasks were better when women had lower progesterone levels. Women in the follicular stage showed higher emotion recognition accuracy than their midluteal phase counterparts. Women were found to react more to negative stimuli when in midluteal stage over the women in the follicular stage, perhaps indicating more reactivity to social stress during that menstrual cycle phase. Overall, it has been found that women in the follicular phase demonstrated better performance in tasks that contain empathetic traits.

Fear response in women during two different points in the menstrual cycle has been examined. When estrogen is highest in the preovulatory stage, women are significantly better at identifying expressions of fear than women who were menstruating, which is when estrogen levels are lowest. The women were equally able to identify happy faces, demonstrating that the fear response was a more powerful response. To summarize, menstrual cycle phase and the estrogen levels correlates with women’s fear processing.

However, the examination of daily moods in women with measuring ovarian hormones may indicate a less powerful connection. In comparison to levels of stress or physical health, the ovarian hormones had less of an impact on overall mood. This indicates that while changes of ovarian hormones may influence mood, on a day-to-day level it does not influence mood more than other stressors do.

Sexual feelings and behaviors change during the menstrual cycle. Before and during ovulation, high levels of estrogen and androgens result in women having an increased interest in sexual activity. Unlike other animal species, women show interest in sex across all days of the menstrual cycle, regardless of fertility.

Behavior towards potential mating partners changes during different phases of the menstrual cycle. Near ovulation, women may have increased physical attraction and interest in attending social gatherings with men. During the fertile phase of the cycle, women appear to prefer males who are more masculine. The intensity of mate guarding differs across the phases of the cycle, with increased mate guarding occurring when women are fertile.

During the fertile phase, many women experience more attraction, fantasies and sexual interest for extra pair men but not for the primary partner. They also engage in extra-pair flirtations and demonstrate a preference for extra pair copulation.

Preferences for voice pitch change across the cycle. When seeking a short term mating partner, women may prefer a male with a low voice pitch, particularly during the fertile phase. During the late follicular phase, it is common for women demonstrate a preference for mates with a masculine, deep voice. Research has also been conducted on the attractiveness of the female voice throughout the cycle. During their most fertile phase of the menstrual cycle, there is some evidence that female voices are rated as significantly more attractive. This effect is not found with women on the birth control pill.

Women's preference for male's body odor can change across the menstrual cycle. Males who score highly on dominance have been rated as sexier by females during the fertile phase of the menstrual cycle. Additionally, during their most fertile phase of the menstrual cycle, women may show preference for the odor of symmetrical men. This effect is not found for women on the birth control pill. Also, during the late follicular and ovulatory phases, women prefer the scent of masculine men. The scent of androsterone (responsible for testosterone levels) is highly preferred by women during the peak of their fertility in the menstrual cycle. Moreover, women may demonstrate preference for men with a scent that indicates developmental stability.

With regard to women's smell across the cycle, some evidence indicates that men use olfactory cues in order to know if a woman is ovulating. Using a rating of women's odors, women who are ovulating have been rated as more attractive by men. Men demonstrate preferences for the scent of fertile women.

Preferences for facial features in mates can also change across the cycle. There has been no difference found in preference for long-term mating partners during the menstrual cycle; however, those seeking a short-term relationship were more likely to choose a partner with more masculine features than usual. This was found to be the case especially during the woman's high conception risk stage and when salivary testosterone was high. However, when women are in the luteal (non-fertile) phase, they tend to prefer men (and females) with more feminine faces. A preference is also shown for self-resembling faces and apparent health in faces during the luteal phase of the cycle. Apparent health preferences were found to be strongest when progesterone levels were high. Additionally, during the fertile phase, many women show a preference for men with darker skin pigmentation. Research on facial symmetry is mixed.

Preferences for body features can change during the fertile phase of the cycle. Women seeking a short-term partner demonstrate a preference for taller and muscular males. Women also show preferences of males with masculine bodies at peak fertility. Mixed research has been found regarding body symmetry preferences throughout different phases of the cycle.

In short term mates, during the fertile phase, women may show more attraction to dominant men who display social presence. For long-term mates, shifts in desired trait preferences do not occur throughout the cycle.

Females have been found to experience different eating habits at different stages of their menstrual cycle, with food intake being higher during the luteal phase than the follicular phase. Food intake increases by approximately 10% during the luteal phase compared to the follicular phase.

Various studies have shown that during the luteal phase woman consume more carbohydrates, proteins and fats and that 24-hour energy expenditure shows increases between 2.5-11.5%. The increasing intake during the luteal phase may be related to higher preferences for sweet and fatty foods, which occurs naturally and is enhanced during the luteal phases of the menstrual cycle. This is due to the higher metabolic demand during this phase. In particular, women tend to show a cravings for chocolate, with higher cravings during the luteal phase.

Females with premenstrual syndrome (PMS) report changes in appetite across the menstrual cycle more than non-sufferers of PMS, possibly due to their oversensitivity to changes in hormone levels. In women with PMS, food intake is higher in the luteal phase than follicular. The remaining symptoms of PMS, including mood changes and physical symptoms, also occur during the luteal phase. No difference for preference of food types has been found between PMS sufferers and non-sufferers.

The different levels of ovarian hormones at different stages of the cycle have been used to explain eating behaviour changes. Progesterone has been shown to promote fat storage, causing a higher intake of fatty foods during the luteal phase when progesterone levels are higher. Additionally, with a high estrogen level dopamine is ineffective in converting to noradrenaline, a hormone which promotes eating, therefore decreasing appetite. In humans, the level of these ovarian hormones during the menstrual cycle have been found to influence binge eating.

It is theorized that the use of birth control pills should affect eating behaviour as they minimise or remove the fluctuations in hormone levels. The neurotransmitter serotonin is also thought to play a role in food intake. Serotonin is responsible for inhibiting eating and controlling meal size, among other things, and is modulated in part by ovarian hormones.

A number of factors affect whether dieting will affect these menstrual processes: age, weight loss and the diet itself. First, younger women are likely to experience menstrual irregularities due to their diet. Second, menstrual abnormalities are more likely with more weight loss. For example, anovulatory cycles can occur as a result of adopting a restricted diet, as well as engaging in a high amount of exercise. Finally, the cycle is affected more by a vegetarian diet compared to a non-vegetarian diet.

Studies investigating effects of the menstrual cycle on alcohol consumption have found mixed evidence. However, some evidence suggests that individuals consume more alcohol during the luteal stage, especially if these individuals are heavy drinkers or have a family history of alcohol abuse.

The level of substance abuse increases with PMS, mostly with addictive substances such as nicotine, tobacco and cocaine. One theory behind this suggests this higher level of substance abuse is due to decreased self-control as a result of the higher metabolic demands during the luteal phase.

Infrequent or irregular ovulation is called "oligoovulation". The absence of ovulation is called "anovulation". Normal menstrual flow can occur without ovulation preceding it: an anovulatory cycle. In some cycles, follicular development may start but not be completed; nevertheless, estrogens will be formed and stimulate the uterine lining. Anovulatory flow resulting from a very thick endometrium caused by prolonged, continued high estrogen levels is called "estrogen breakthrough bleeding". Anovulatory bleeding triggered by a sudden drop in estrogen levels is called withdrawal bleeding. Anovulatory cycles commonly occur before menopause (perimenopause) and in women with polycystic ovary syndrome.

Very little flow (less than 10 ml) is called "hypomenorrhea". Regular cycles with intervals of 21 days or fewer are "polymenorrhea"; frequent but irregular menstruation is known as "metrorrhagia". Sudden heavy flows or amounts greater than 80 ml are termed "menorrhagia". Heavy menstruation that occurs frequently and irregularly is "menometrorrhagia". The term for cycles with intervals exceeding 35 days is "oligomenorrhea". Amenorrhea refers to more than three to six months without menses (while not being pregnant) during a woman's reproductive years.
The term for painful periods is "Dysmenorrhea".

The menstrual cycle can be described by the ovarian or uterine cycle. The ovarian cycle describes changes that occur in the follicles of the ovary whereas the uterine cycle describes changes in the endometrial lining of the uterus. Both cycles can be divided into three phases. The ovarian cycle consists of the follicular phase, ovulation, and the luteal phase, whereas the uterine cycle consists of menstruation, proliferative phase, and secretory phase.

The follicular phase is the first part of the ovarian cycle. During this phase, the ovarian follicles mature and get ready to release an egg. The latter part of this phase overlaps with the proliferative phase of the uterine cycle.

Through the influence of a rise in follicle stimulating hormone (FSH) during the first days of the cycle, a few ovarian follicles are stimulated. These follicles, which were present at birth and have been developing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. Under the influence of several hormones, all but one of these follicles will stop growing, while one dominant follicle in the ovary will continue to maturity. The follicle that reaches maturity is called a tertiary or Graafian follicle, and it contains the ovum.

Ovulation is the second phase of the ovarian cycle in which a mature egg is released from the ovarian follicles into the oviduct. During the follicular phase, estradiol suppresses release of luteinizing hormone (LH) from the anterior pituitary gland. When the egg has nearly matured, levels of estradiol reach a threshold above which this effect is reversed and estrogen stimulates the production of a large amount of LH. This process, known as the LH surge, starts around day 12 of the average cycle and may last 48 hours.

The exact mechanism of these opposite responses of LH levels to estradiol is not well understood. In animals, a gonadotropin-releasing hormone (GnRH) surge has been shown to precede the LH surge, suggesting that estrogen's main effect is on the hypothalamus, which controls GnRH secretion. This may be enabled by the presence of two different estrogen receptors in the hypothalamus: estrogen receptor alpha, which is responsible for the negative feedback estradiol-LH loop, and estrogen receptor beta, which is responsible for the positive estradiol-LH relationship. However, in humans it has been shown that high levels of estradiol can provoke 32 increases in LH, even when GnRH levels and pulse frequencies are held constant, suggesting that estrogen acts directly on the pituitary to provoke the LH surge.

The release of LH matures the egg and weakens the wall of the follicle in the ovary, causing the fully developed follicle to release its secondary oocyte. If it is fertilized by a sperm, the secondary oocyte promptly matures into an ootid and then becomes a mature ovum. If it is not fertilized by a sperm, the secondary oocyte will degenerate. The mature ovum has a diameter of about 0.2 mm.

Which of the two ovaries—left or right—ovulates appears essentially random; no known left and right co-ordination exists. Occasionally, both ovaries will release an egg; if both eggs are fertilized, the result is fraternal twins.

After being released from the ovary, the egg is swept into the fallopian tube by the fimbria, which is a fringe of tissue at the end of each fallopian tube. After about a day, an unfertilized egg will disintegrate or dissolve in the fallopian tube.

Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla, the widest section of the fallopian tubes. A fertilized egg immediately begins the process of embryogenesis, or development. The developing embryo takes about three days to reach the uterus and another three days to implant into the endometrium. It has usually reached the blastocyst stage at the time of implantation.

In some women, ovulation features a characteristic pain called "mittelschmerz" (German term meaning "middle pain"). The sudden change in hormones at the time of ovulation sometimes also causes light mid-cycle blood flow.

The luteal phase is the final phase of the ovarian cycle and it corresponds to the secretory phase of the uterine cycle. During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone. The increased progesterone in the adrenals starts to induce the production of estrogen. The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. Consequently, the level of FSH and LH fall quickly over time, and the corpus luteum subsequently atrophies. Falling levels of progesterone trigger menstruation and the beginning of the next cycle. From the time of ovulation until progesterone withdrawal has caused menstruation to begin, the process typically takes about two weeks, with 14 days considered normal. For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle.

The loss of the corpus luteum is prevented by fertilization of the egg. The syncytiotrophoblast, which is the outer layer of the resulting embryo-containing structure (the blastocyst) and later also becomes the outer layer of the placenta, produces human chorionic gonadotropin (hCG), which is very similar to LH and which preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy. Most pregnancy tests look for the presence of hCG.

The uterine cycle has three phases: menses, proliferative, secretory.

Menstruation (also called menstrual bleeding, menses, catamenia or a period) is the first phase of the uterine cycle. The flow of menses normally serves as a sign that a woman has not become pregnant. (However, this cannot be taken as certainty, as a number of factors can cause bleeding during pregnancy; some factors are specific to early pregnancy, and some can cause heavy flow.)

"Eumenorrhea" denotes normal, regular menstruation that lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal). The average blood loss during menstruation is 35 milliliters with 10–80 ml considered normal. Women who experience Menorrhagia are more susceptible to iron deficiency than the average person. An enzyme called plasmin inhibits clotting in the menstrual fluid.

Painful cramping in the abdomen, back, or upper thighs is common during the first few days of menstruation. Severe uterine pain during menstruation is known as dysmenorrhea, and it is most common among adolescents and younger women (affecting about 67.2% of adolescent females). When menstruation begins, symptoms of premenstrual syndrome (PMS) such as breast tenderness and irritability generally decrease. Many sanitary products are marketed to women for use during their menstruation.

The proliferative phase is the second phase of the uterine cycle when estrogen causes the lining of the uterus to grow, or proliferate, during this time. As they mature, the ovarian follicles secrete increasing amounts of estradiol, and estrogen. The estrogens initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. The estrogen also stimulates crypts in the cervix to produce fertile cervical mucus, which may be noticed by women practicing fertility awareness.

The secretory phase is the final phase of the uterine cycle and it corresponds to the luteal phase of the ovarian cycle. During the secretory phase, the corpus luteum produces progesterone, which plays a vital role in making the endometrium receptive to implantation of the blastocyst and supportive of the early pregnancy, by increasing blood flow and uterine secretions and reducing the contractility of the smooth muscle in the uterus; it also has the side effect of raising the woman's basal body temperature.

While some forms of birth control do not affect the menstrual cycle, hormonal contraceptives work by disrupting it. Progestogen negative feedback decreases the pulse frequency of gonadotropin-releasing hormone (GnRH) release by the hypothalamus, which decreases the release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) by the anterior pituitary. Decreased levels of FSH inhibit follicular development, preventing an increase in estradiol levels. Progestogen negative feedback and the lack of estrogen positive feedback on LH release prevent a mid-cycle LH surge. Inhibition of follicular development and the absence of a LH surge prevent ovulation.

The degree of ovulation suppression in progestogen-only contraceptives depends on the progestogen activity and dose. Low dose progestogen-only contraceptives—traditional progestogen only pills, subdermal implants Norplant and Jadelle, and intrauterine system Mirena—inhibit ovulation in about 50% of cycles and rely mainly on other effects, such as thickening of cervical mucus, for their contraceptive effectiveness. Intermediate dose progestogen-only contraceptives—the progestogen-only pill Cerazette and the subdermal implant Nexplanon—allow some follicular development but more consistently inhibit ovulation in 97–99% of cycles. The same cervical mucus changes occur as with very low-dose progestogens. High-dose, progestogen-only contraceptives—the injectables Depo-Provera and Noristerat—completely inhibit follicular development and ovulation.

Combined hormonal contraceptives include both an estrogen and a progestogen. Estrogen negative feedback on the anterior pituitary greatly decreases the release of FSH, which makes combined hormonal contraceptives more effective at inhibiting follicular development and preventing ovulation. Estrogen also reduces the incidence of irregular breakthrough bleeding. Several combined hormonal contraceptives—the pill, NuvaRing, and the contraceptive patch—are usually used in a way that causes regular withdrawal bleeding. In a normal cycle, menstruation occurs when estrogen and progesterone levels drop rapidly. Temporarily discontinuing use of combined hormonal contraceptives (a placebo week, not using patch or ring for a week) has a similar effect of causing the uterine lining to shed. If withdrawal bleeding is not desired, combined hormonal contraceptives may be taken continuously, although this increases the risk of breakthrough bleeding.

Breastfeeding causes negative feedback to occur on pulse secretion of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH). Depending on the strength of the negative feedback, breastfeeding women may experience complete suppression of follicular development, but no ovulation, or normal menstrual cycle may resume. Suppression of ovulation is more likely when suckling occurs more frequently. The production of prolactin in response to suckling is important to maintaining lactational amenorrhea. On average, women who are fully breastfeeding whose infants suckle frequently experience a return of menstruation at fourteen and a half months postpartum. There is a wide range of response among individual breastfeeding women, however, with some experiencing return of menstruation at two months and others remaining amenorrheic for up to 42 months postpartum.

The word "menstruation" is etymologically related to "moon". The terms "menstruation" and "menses" are derived from the Latin "mensis" (month), which in turn relates to the Greek "mene" (moon) and to the roots of the English words "month" and "moon".

Even though the average length of the human menstrual cycle is similar to that of the lunar cycle, in modern humans there is no relation between the two. The relationship is believed to be a coincidence. Light exposure does not appear to affect the menstrual cycle in humans. A meta-analysis of studies from 1996 showed no correlation between the human menstrual cycle and the lunar cycle, nor did data analysed by period-tracking app Clue, submitted by 1.5m women, of 7.5m menstrual cycles.

Dogon villagers did not have electric lighting and spent most nights outdoors, talking and sleeping, so they were apparently an ideal population for detecting a lunar influence; none was found.

In a number of countries, mainly in Asia, legislation or corporate practice has introduced formal menstrual leave to provide women with either paid or unpaid leave of absence from their employment while they are menstruating. Countries with policies include Japan, Taiwan, Indonesia, and South Korea. The practice is controversial due to concerns that it bolsters the perception of women as weak, inefficient workers, as well as concerns that it is unfair to men.

While the normal menstrual cycle in the human typically lasts 4 weeks (28 days, range 24–35 days) and consists of a follicular phase, ovulation, and a luteal phase, followed by either menstruation or pregnancy, the anovulatory cycle has cycle
lengths of varying degrees. In many circumstances, menstrual intervals are prolonged exceeding 35 days leading to oligomenorrhea (cycle >35 – 180 days interval), or even longer, amenorrhea. In other cases, menstruation may be fairly regular (eumenorrhea), or more frequent (intervals < 21 days), or there may be a loss of menstrual pattern (menorrhagia, dysfunctional uterine bleeding).

Anovulatory cycle

The anovulatory cycle is a menstrual cycle characterized by varying degrees of menstrual intervals and the absence of ovulation and a luteal phase. In the absence of ovulation, there will be infertility.

Normal menstrual bleeding in the ovulatory cycle is a result of a decline in progesterone due to the demise of the corpus luteum. It is thus a progesterone withdrawal bleeding. As there is no progesterone in the anovulatory cycle, bleeding is caused by the inability of estrogen — that needs to be present to stimulate the endometrium in the first place — to support a growing endometrium. Anovulatory bleeding is hence termed 'estrogen breakthrough bleeding.A physician needs to investigate the cause of anovulation. Common causes are:

With excessive or prolonged bleeding the diagnosis has to be made by a physician on a speedy basis. Other causes of gynecological bleeding need to be excluded, specifically bleeding related to pregnancy, leiomyoma, and cancer of the cervix or uterus.

Women who do not ovulate and who want to get pregnant need a medical work-up to find out why they do not ovulate. Drugs are often given to induce ovulation, including oral medication such as clomiphene or injectable medications.
In patients who do not want to get pregnant anovulation can be managed with the use of cyclic progesterone or progestin supplementation or use of hormonal contraception.