The menstrual cycle

Menstruation is the periodic discharge of blood and cells from the lining of a non-pregnant uterus in human females and some non-human primates, such as chimpanzees, gorillas, and orangutans. Menstruation begins at puberty and ends with menopause. The menstrual cycle is the entire, recurring cycle of physiological changes in females that is associated with reproductive fertility, while menstruation is the part of that cycle that involves the shedding of the uterine lining between ovulations.

Unlike many animals in which sexual activity is restricted to near the time of ovulation, the human menstrual cycle allows reproduction year round and lacks overt signs of the time of ovulation. Such a system fosters receptivity to sexual intimacy at any time, thus promoting pair bonding and allowing sex, considered by many religious traditions as ideally a holy act, to encourage and cement the bonds of love between husband and wife.

This article concentrates on the menstrual cycle as it occurs in human beings.



Human beings and the great apes (chimpanzees, gorillas, and orangutans) experience a true menstrual cycle. Old World Monkeys (baboons, macaques, etc.) are sometimes included as having a menstrual cycle depending on how terms are defined. However, most placental mammals—such as dogs, cats, elephants, and New World Monkeys (howler, capuchin, and spider monkeys, etc.)—experience estrus instead. One difference is that animals that have estrous cycles reabsorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead. Another difference is sexual activity. In species with estrous cycles, females are generally only sexually active during the estrous phase of their cycle (near the time of ovulation). This is also referred to as being "in heat." In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even when they are not about to ovulate. Humans, unlike some other species, do not have any external signs to signal receptivity at ovulation.

The menstrual cycle is under the control of the reproductive hormone system and is necessary for reproduction. In women, menstrual cycles occur typically on a monthly basis between puberty and menopause.

During the menstrual cycle, the sexually mature female body builds up the lining of the uterus with gradually increasing amounts of estrogen, and when this hormone reaches a critical level, estradiol is produced, and shortly thereafter there is the stimulation of the ovaries with follicle stimulating hormone (FSH), and luteinizing hormone (LH). Follicles (a group of cells) begin developing, and within a few days one "matures" into an ovum, or egg. The ovary then releases this egg, (or occasionally two, which might result in dizygotic, or non-identical, twins) at the time of ovulation. The lining of the uterus, called the endometrium, peaks shortly there afterward in a synchronized fashion. After ovulation, this lining changes to prepare for potential conception and implantation of the fertilized egg to establish a pregnancy. The hormone progesterone rises after ovulation, and peaks shortly afterwards.

If fertilization and pregnancy do not ensue, the uterus sheds the lining, culminating with menstruation, which marks the low point for estrogen activity. This manifests itself to the outer world in the form of the menses (also menstruum): essentially part of the endometrium and blood products that pass out of the body through the vagina. Although this is commonly referred to as blood, it differs in composition from venous blood. Menstrual cycles are counted from the beginning of menstruation because this is an outside sign that corresponds closely with the hormonal cycle. Menses, or bleeding, and other menstrual signs may end at different points in the new cycle.

Common terminology refers to menstruation and menses as a period. This bleeding serves as a sign that a woman has not become pregnant. However, this cannot be taken as certainty, as sometimes there is some bleeding in early pregnancy. During the reproductive years, failure to menstruate may provide the first indication to a woman that she may have become pregnant. A woman might say that her "period is late" when an expected menstruation has not started and she might have become pregnant.

Menstruation forms a normal part of a natural cyclic process occurring in healthy women between puberty and the end of the reproductive years. The onset of menstruation, known as menarche, occurs at an average age of 12, but is normal anywhere between the ages of eight and 16. Factors such as heredity, diet, and overall health can accelerate or delay the onset of menarche (Menstruation 2005).

The condition precocious puberty has caused menstruation to occur in girls as young as eight months old (Mikkelson 2004). Some women experience their first period in their late teens. The last period, menopause, usually occurs between the ages of 45 and 55. Deviations from this pattern deserve medical attention. Amenorrhea refers to a prolonged absence of menses during the reproductive years of a woman for reasons other than pregnancy. For example, women with very low body fat, such as athletes, may cease to menstruate. The presence of menstruation does not prove that ovulation took place; women who do not ovulate may have menstrual cycles. Those anovulatory cycles tend to take place less regularly and show greater variation in cycle length. In addition, the absence of menstruation also does not prove that ovulation did not take place since hormone disruptions in non-pregnant women can suppress bleeding on occasion.

The Normal Menstrual Cycle in Humans

Women show considerable variation in the lengths of their menstrual cycles. While cycle length may vary, 28 days is generally taken as representative of the average ovulatory cycle in women. Convention uses the onset of menstrual bleeding to mark the beginning of the cycle, so the first day of bleeding is called "cycle day one."

The menstrual cycle can be divided into four phases:


Eumenorrhea denotes normal, regular menstruation that lasts for a few days (usually three to five days, but anywhere from two to seven days is considered normal) (National Women's Health Information Center 2002). The average blood loss during menstruation is 35 milliliters with 10-80 mL considered normal (Healy 2004); many women also notice shed endometrium lining that appears as tissue mixed with the blood. An enzyme called plasmin—contained in the endometrium—inhibits the blood from clotting. Because of this blood loss, women have higher dietary requirements for iron than do males to prevent iron deficiency. Many women experience uterine cramps, also referred to as dysmenorrhea, during this time. A vast industry has grown to provide sanitary products to help women to manage their menses.

Follicular Phase

Through the influence of a rise in Follicle stimulating hormone (FSH), five to seven tertiary-stage ovarian follicles are recruited for entry into the menstrual cycle. These follicles, which have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. In a signal cascade kicked off by luteinizing hormone (LH), the follicles secrete estradiol, a steroid that acts to inhibit pituitary secretion of FSH. With diminished FSH supply comes a slowing in growth that eventually leads to follicle death, known as atresia. The largest follicle secretes inhibin that serves as a finishing blow to less competent follicles by further suppressing FSH. This dominant follicle continues growing, forms a bulge near the surface of the ovary, and soon becomes competent to ovulate.

The follicles also secrete estrogens (of which estradiol is a member). Estrogens initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. If fertilized, the embryo will implant itself within this hospitable flesh.


The ovary about to release an egg

When the follicle has matured, it secretes enough estradiol to trigger the acute release of luteinizing hormone (LH). In the average cycle this LH surge starts around cycle day 12 and may last 48 hours. The release of LH matures the egg and weakens the wall of the follicle in the ovary. This process leads to ovulation: the release of the now mature ovum, the largest cell of the body (with a diameter of about 0.5 mm). Which of the two ovaries—left or right—ovulates appears essentially random; no known left/right co-ordination exists. The fallopian tubes, which connect the ovaries to the uterus, need to capture the egg and provide the site for fertilization. A characteristic clear and stringy mucus exhibiting spinnbarkeit develops at the cervix, ready to accept sperm from intercourse. Spinnbarkeit is a medical term and refers to the stringy and/or stretchy quality found to varying degrees in mucus, saliva, and similar fluids. In some women, ovulation features a characteristic pain called Mittelschmerz (German term meaning 'middle pain'), which lasts for several hours. The sudden change in hormones at the time of ovulation also causes light mid-cycle bleeding for some women. Many women perceive the vaginal and cervical mucus changes at ovulation, particularly if they are monitoring themselves for signs of fertility. An unfertilized egg will eventually disintegrate or dissolve in the uterus. Scientific investigations have indicated that the olfactory acuity, or the sense of smell, is greatest in women during ovulation (Navarrete-Palacios 2003).

Luteal Phase

The most apt time to become pregnant is when the egg is in the place where it can be fertilized easily. The corpus luteum is the solid body formed in the ovaries after the egg has been released from the fallopian tube and that continues to grow and divide for a while. After ovulation, the residual follicle transforms into thecorpus luteum under the support of the pituitary hormones. This corpus luteum will produce progesterone in addition to estrogens for approximately the next two weeks. Progesterone plays a vital role in converting the proliferative endometrium into a secretory lining receptive for implantation and supportive of the early pregnancy. It raises the body temperature by one-half to one whole degree Fahrenheit (one-quarter to one-half degree Celsius); thus, women who record their temperature on a daily basis will notice that they have entered the luteal phase.

If fertilization of an egg has occurred, it will travel as an early embryo through the fallopian tube to the uterine cavity and implant itself six to 12 days after ovulation. Shortly after implantation, the growing embryo will signal its existence to the maternal system. One very early signal consists of human chorionic gonadotropin (hCG), a hormone that pregnancy tests can measure. This signal has an important role in maintaining the corpus luteum and enabling it to continue to produce progesterone. In the absence of a pregnancy and without hCG, the corpus luteum demises and inhibin and progesterone levels fall. This will set the stage for the next cycle. Progesterone withdrawal leads to menstrual shedding (progesterone withdrawal bleeding), and falling inhibin levels allow FSH levels to rise to raise a new crop of follicles.

Menstrual Symptoms

In many women, various unpleasant symptoms caused by the involved hormones and by cramping of the uterus can precede or accompany menstruation. More severe symptoms may include significant menstrual pain (dysmenorrhea), abdominal pain, migraine headaches, depression, and irritability. Some women encounter premenstrual stress syndrome (PMS or premenstrual syndrome), severe cases of which are clinically referred to as premenstrual dysphoric disorder (PMDD). Other women are said to suffer from what some doctors call post-menstrual syndrome, where similar symptoms manifest themselves. This is rare and is usually not as severe or as long as pre-menstrual syndrome. Breast discomfort caused by premenstrual water retention is very common. The list of symptoms experienced varies from person to person. Furthermore, within an individual, the severity of the symptoms may vary from cycle to cycle. Pharmaceutical and herbal companies provide menstrual products designed to lessen or relieve some or all of these symptoms.

The Fertile Window

The length of the follicular phase—and consequently the length of the menstrual cycle—may vary widely. The luteal phase, however, almost always takes the same number of days. Some women have a luteal phase of ten days, others of 16 days (the average is 14 days), but for each individual woman, this length will remain constant. Sperm survive inside a woman for three days on average, with survival time up to five days considered normal. A pregnancy resulting from sperm life of eight days has been documented (Ball 1977, Billings 2005, Sinha 1993).

The most fertile period (the time with the highest likelihood of sexual intercourse leading to pregnancy) covers the time from some five days before ovulation until 1-2 days after ovulation. In an average 28-day cycle with a 14-day luteal phase, this corresponds to the second and the beginning of the third week of the cycle.

Fertility awareness methods of birth control attempt to determine the precise time of ovulation in order to find the relatively fertile and the relatively infertile days in the cycle.

People who have heard about the menstrual cycle and ovulation may commonly and mistakenly assume, for contraceptive purposes, that menstrual cycles always take a regular 28 days, and that ovulation always occurs 14 days after beginning of the menses. This assumption may lead to unintended pregnancies. Note, too, that not every bleeding event counts as a menstruation, and this can mislead people in their calculation of the fertile window.

If a woman wants to conceive, the most fertile time occurs between 19 and 10 days prior to the expected menses. Many women use ovulation detection kits that detect the presence of the LH surge in the urine to indicate the most fertile time. Other ovulation detection systems rely on observation of one or more of the three primary fertility signs (basal body temperature, cervical fluid, and cervical position).

Among women living closely together, the onsets of menstruation may tend to synchronize somewhat, an apparent example of entrainment—an innate physiological process whereby connected systems with similar periods fall into synchrony. Such a process likewise is seen in the turning of plants toward sunlight and the matching of human sleep schedules with their circadian rhythm. This McClintock effect of menstruation was first described in 1971, and possibly explained by the action of pheromones in 1998 (Stern 1998). However, subsequent research has called this conclusion into question.

Hormonal Control

Extreme intricacies regulate the menstrual cycle. For many years, researchers have argued over which regulatory system has ultimate control: the hypothalamus, the pituitary gland, or the ovary with its growing follicle; but all three systems have to interact.

In any scenario, the growing follicle has a critical role: it matures the lining, provides the appropriate feedback to the hypothalamus and pituitary, and modifies the mucus changes at the cervix. Two sex hormones play a role in the control of the menstrual cycle: estradiol and progesterone. While estrogen peaks twice, during follicular growth and during the luteal phase, progesterone remains virtually absent prior to ovulation, but becomes critical in the luteal phase and during pregnancy. Many tests for ovulation check for the presence of progesterone. These sex hormones come under the influence of the pituitary gland, and both FSH and LH play necessary roles. FSH stimulates immature follicles in the ovaries to grow. LH triggers ovulation. The gonadotropin-releasing hormone of the hypothalamus controls the pituitary gland, yet both the pituitary and the hypothalamus receive feedback from the follicle. After ovulation the corpus luteum—which develops from the burst follicle and remains in the ovary—secretes both estradiol and progesterone. Only if pregnancy occurs do hormones appear in order to suspend the menstrual cycle, while production of estradiol and progesterone continues. Abnormal hormonal regulation leads to disturbance in the menstrual cycle.

Some women with neurological conditions experience increased activity of their conditions at about the same time every month. Eighty percent of women with epilepsy (tendency to have seizures) have more seizures than usual in the phase of their cycle when progesterone declines and estrogen increases.

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 (Maguire 2005).

Hidden Ovulation

Unlike almost all other species, the external physical changes of a human female near ovulation are very subtle. A woman may sense her own ovulation while it may remain indiscernible to others; this is considered to have socio-biological significance. In contrast, other species often signal receptivity through heat. The great apes are the only other mammals to have hidden ovulation.

The Ovary as an Egg-Bank

Evidence suggests that eggs are formed from germ cells early in fetal life. The number is reduced to an estimated 400,000 to 450,000 immature eggs residing in each ovary at puberty. The menstrual cycle, as a biologic event, allows for ovulation of one egg typically each month. Thus, over her lifetime a woman will ovulate approximately 400 to 450 times. All the other eggs dissolve by a process called atresia. 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. This possibility is supported by the observation that fetuses and infants of older mothers have higher rates of chromosome abnormalities than those of older fathers.

The Anovulatory Menstrual Cycle

Not all menstruations result from an ovulatory menstrual cycle (anovulatory cycle- literally 'an-' absence of 'ovulation'). In some women, follicular development may start but not complete. Nevertheless, estrogens will form and will stimulate the uterine lining. Sooner or later, the uterus will shed this lining. As no ovulation and no progesterone involvement occurs, doctors call this type of bleeding an “estrogen breakthrough bleeding” and cannot always predict its duration or frequency. Anovulatory bleeding commonly occurs prior to menopause or in women with polycystic ovary syndrome (hormonal reproductive problem most commonly found in women of childbearing age). Infrequent or irregular ovulation is called oligoovulation.

Cycle Abnormalities


The "normal menstrual cycle" occurs every 28 days ± 7 days.

The medical term for cycles with intervals of 21 days or fewer is polymenorrhea. On the other hand, the term for cycles with intervals exceeding 35 days is oligomenorrhea (or amenorrhea if intervals exceed 180 days).


The normal menstrual flow amounts to 50 ml ± 30 ml. It follows a "crescendo-decrescendo" pattern; that is, it starts at a moderate level, increases somewhat, and then slowly tapers. Sudden heavy flows or amounts in excess of 80 ml (hypermenorrhea or menorrhagia) may stem from hormonal disturbance, uterine abnormalities, including uterine leiomyoma (benign tumor) or cancer, and other causes. Doctors call the opposite phenomenon of bleeding very little hypomenorrhea.


The typical woman bleeds ("is on her period") for three to seven days out of each month.

Prolonged bleeding (metrorrhagia, also meno-metrorrhagia) no longer shows a clear interval pattern. Dysfunctional uterine bleeding refers to hormonally-caused bleeding abnormalities, typically anovulation.

All these bleeding abnormalities need medical attention; they may indicate hormone imbalances, uterine fibroids, or other problems. As pregnant patients may bleed, a pregnancy test forms part of the evaluation of abnormal bleeding.

The Birth Control Pill

Estrogens and progesterone-like hormones make up the main active ingredients of birth control pills. Typically, they tend to mimic a menstrual cycle in appearance, but suppress the critical event of the ovulatory cycle, namely ovulation. Normally, a woman takes hormone pills for 21 days, followed by seven days of non-functional placebo sugar pills or no pills at all. Then the cycle starts again. During the seven placebo days, a withdrawal bleeding occurs; this differs from ordinary menstruation, and skipping the placebos and continuing with the next batch of hormone pills may suppress it. (Two main versions of the pill exist: monophasic and triphasic. With triphasic pills, skipping of the placebos and continuing with the next month's dose can make a woman more likely to experience spotting or breakthrough bleeding.) In 2003, the United States Food and Drug Administration (FDA) approved low-dose monophasic birth control pills that induce withdrawal bleedings only every 3 months.

Etymology and the Lunar Month

The terms "menstruation" and "menses" come 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—reflecting the fact that the moon also takes close to 28 days to revolve around the Earth (actually 27.32 days). The synodical lunar month, the period between two new moons (or full moons), is 29.53 days long.

A 1975 book by Louise Lacey documented the experience of herself and 27 of her friends, who found that when they removed all artificial night lighting their menstrual cycles began to occur in rhythm with the lunar cycle. She dubbed the technique Lunaception (Lacey 1975). Later studies in both humans (Singer 2004) and animals (Harder 2004) have found that artificial light at night does influence the menstrual cycle in humans and the estrus cycle in mice (usually making it more irregular), though none have duplicated the synchronization of women's menstrual cycles with the lunar cycle. One author has suggested that sensitivity of women's cycles to night lighting is caused by nutritional deficiencies of certain vitamins and minerals (Shannon 2001).

Some have suggested that the fact that other animals' menstrual cycles appear to be greatly different from lunar cycles (see Menstruation#Menstruation in Other Mammals below) supports the view that the average length of humans' cycle is likely a coincidence (Adams 1999, Cutler 1980).

Menstrual Products

While some women allow their menses to flow freely or learn to recognize when their menses will flow, most women prefer to use some artificial means to absorb or catch their menses to prevent soiling their clothes. There are a number of different methods used:

  • Sanitary towels, sanitary napkins, or pads—somewhat rectangular pieces of material worn in the underpants to absorb menstrual flow, often with "wings," pieces that fold around the panties, and/or an adhesive backing to hold the pad in place. Reusable cloth pads are made of cotton (often organic), terrycloth, or flannel, and may be hand-sewn (from material or reused old clothes and towels) or store bought. Disposable synthetic pads are made of wood pulp or synthetic products, usually with a plastic lining and bleached.
  • Tampons—Disposable wads of treated rayon/cotton blends or all-cotton fleece, usually bleached, that are inserted into the vagina to absorb menstrual flow.
  • Menstrual cups—A firm, flexible cup- or bell-shaped device worn inside the vagina to catch menstrual flow. Reusable versions include rubber or silicone cups. Disposable versions come in soft plastic cups.
  • Sea sponges—Reusable soft sponges from plant-like animals that grow on the ocean floor, worn internally to absorb blood.
  • Padettes—Disposable wads of treated rayon/cotton blend fleece that are placed within the inner labia (part of the external female genetilia) to absorb menstrual flow.
  • Padded panties—Reusable cloth (usually cotton) undergarment/underwear with extra absorbent layers sewn in to absorb flow.
  • Blanket, towel—Large reusable piece of cloth, most often used at night, placed between legs to absorb menstrual flow.

Pharmaceutical companies also provide products—commonly non-steroidal anti-inflammatory drugs (NSAIDs)—to relieve menstrual cramps.


Much debate centers around which menstrual products to use. The main debate can be summarized as one between the convenience, availability, and general knowledge of disposables versus the environmental, monetary, and potential health benefits of reusables. A secondary aspect of this is commercial responsibility. Disposable menstrual products compose a large and powerful industry in the West, with a near monopoly on advertising, supermarket shelves, and menstrual education. This leads many people to believe that these corporate products are their only option. Many people object to the negative portrayal of menstruation in advertising; menstruation is portrayed as shameful, unnatural, smelly, and hindering. In contrast, the reusable menstrual products industry is composed mostly of small, independent, and often woman-owned businesses, and often a more positive view of the women's cycle.

A summary of the main issues of debate:

  • Environmental waste: Tampons, pads, disposable cups and their packaging generate tons of bulky waste per year, much of which is not biodegradable.
  • Cost: Many disposables have a cheaper upfront cost than reusables, but over time this cost may be recouped many times over from savings on reusables. Some reusables can be made for free from old clothes or other scraps of cloth.
  • Health concerns:
    1. Bleaching: Many women object to the chlorine bleaching of disposable menstrual products, which leaves trace amounts of dioxin, a carcinogen (cancer causing agent), in their bodies.
    2. Scents and deodorizers: Chemical scents and deodorizers can cause rashes, irritation, and allergic reactions. They can upset the pH balance of the vagina and cause yeast infections.
    3. Specific to tampons: Toxic Shock Syndrome is indirectly caused by staphylococcus aureus, which can thrive in the environment found in tampon fibers. TSS is very rare, with only approximately 40 cases per year in the United Kingdom. One or two out of 100,000 women in the United States are affected by TSS (age range of 15-44 years) (Toxic 2005). Tampon-associated TSS is not caused by the staphylococci invading into tissues, but rather as a result of the release of a protein called toxic shock syndrome toxin, or TSST (Staphylococcus 1996). TSST is absorbed into the body where it acts as a toxin. Toxic Shock Syndrome can, and does, cause death. TSS can be avoided by using the least absorbent tampon possible for one's flow, changing tampons at least every eight hours, and/or by avoiding tampons altogether. This may apply to sea sponges also, though no cases of TSS with sea sponge use have been reported.

Culture and Menstruation

Several religions have menstruation-related rituals.


Some Christian denominations, including some (but by no means all) authorities of the Orthodox Church, advise women not to receive communion during their menstrual period. Other denominations follow the rules laid out in the Holiness Code section of Leviticus, somewhat similar to the Jewish ritual of Niddah.

Most Christian denominations do not follow any specific rituals or rules related to menstruation.


The traditional Islamic interpretation of the Qur'an forbids intercourse, but not physical intimacy, during a woman's menstrual period. During menstrual periods, women are not allowed to perform prayers or fast.


A ritual exclusion applies to a woman while menstruating and for about a week thereafter, until she immerses herself in a mikvah, which is a ritual bath. During this time, a married couple has to avoid sexual intercourse and physical intimacy. While Orthodox Jews follow this exclusion, many Jews in other branches of the religion do not.


During the menses, many Hindu women, especially Brahmins, avoid routine work and do not enter the kitchen or pooja (altar) room. They do not touch members of their family or exchange clothes with them. They may even sleep on mats instead of their usual beds, or in special rooms or huts for this purpose. This is maintained for three days, with the starting day counted as one day. On the morning of the fourth day, they bathe and end the ritual. On the fifth day, they take an oil bath and go back to their regular routine.

As with menstrual regulations in many other religions, this is a rule followed more in orthodox homes than in households following other forms of Hinduism.


In Buddhism (Theravada or Hinayana) menstruation is viewed as "a natural physical excretion that women have to go through on a monthly basis, nothing more or less." However Hindu belief and practice carried over into Thai Buddhist culture.

Ayyavazhi tradition

Almost all the practices of women during their menses in Hindu culture were also practices in Ayyavazhi society. However, they did enter the kitchen and bathe on each day of their menses, in contrast to the Hindu tradition.

Tribal societies

Some tribal societies place restrictions and taboos on menstruating women. For example, Dogon women must stay in a special hut during their menstrual period.


Mystics have sometimes elaborated "equivalencies," analogizing the waxing and waning of the moon with influences on human menstruation. In this astrological context, some women call menstruation their "moon time." Some ancient views also regarded menstruation as a cleansing of the body and compare it to bloodletting as a major medical treatment of pre-modern times. Bloodletting refers to the removal of blood from a vein usually for therapeutic measures.

Menstruation in Other Mammals

A regular menstrual cycle as described here only occurs in the great apes. Menstrual cycles vary in length from an average of 29 days in orangutans to an average of 37 days in chimpanzees.

Females of other mammalian species go through certain episodes called "estrus," or "heat," in each breeding season. During these times, ovulation occurs and females become receptive to mating, a fact advertised to males in some way. If no fertilization takes place, the uterus reabsorbs the endometrium and no menstrual bleeding occurs. Significant differences exist between the estrus and the menstrual cycle. Some animals, such as domestic cats and dogs, do produce a very short and mild menstrual flow. However, due to its small amount (and personal cleanliness in cats) it passes largely unnoticed by pet owners.



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