Menstrual cycle

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The menstrual cycle is the periodic change in a woman's body that occurs every month between puberty and menopause and that relates to reproduction. The average human menstrual cycle, regulated by hormones, takes 28 days, but it can range from 21 to 35 days.


Menstruation, commonly called a woman's period, consists of the approximately monthly shedding of the lining of the uterus; the blood and tissues that have built up throughout the month preparing for fertilization leave the body through the vagina. This bleeding, which can last from 2 to 7 days, generally serves as an indication that conception has not taken place and that the menstruating woman has not become pregnant. The discharged material variously bears the name of menses or (rarely) of menstruum (plural: menstrua).

Menstruation forms a normal part of a natural cyclic process occurring in all healthy adult women between puberty and menopause. The onset of regular menstruation, known as menarche, occurs between the ages of 8 and 18; menopause normally occurs between the ages of 40 and 60.

The cycle

By convention, the onset of menstrual bleeding (menstruation or menses) marks the beginning of the cycle. Menstruation lasts for several days and involves the loss of the lining of the uterus. The uterus prior to menstruation had prepared to accept a fertilized egg (ovum), but none arrived, and so the uterus expels its lining (called endometrium). Therefore, if menstrual bleeding occurs, a woman knows that she is not pregnant.

Then a new egg matures in the ovaries, and about at the middle of the cycle (14 days before beginning of the next menstrual bleeding), ovulation occurs, meaning that the ovary releases an egg, which enters the fallopian tube. In some women, ovulation features a characteristic pain called Mittelschmerz which lasts for several hours. A characteristic clear and stringy mucus develops at the cervix, ready to accept sperm. The egg (with a diameter of about 0.5 mm) travels through the fallopian tube to the uterus, pushed along by movements of the lining of the tube. This trip of about 7–14 cm takes about one day, and fertilization of the egg may occur during this period.

In the meantime, the endometrium has started to grow again. If fertilization occurs, the egg implants itself in the wall of the uterus and the major changes of pregnancy commence, including the suspension of the menstrual cycle for the duration of the pregnancy. If, on the other hand, no fertilization occurs, the endometrium is lost with bleeding, and the cycle starts again.

Menstruation involves the loss of about 50 millilitres of blood. An enzyme called plasmin -- contained in the endometrium -- inhibits the blood from clotting. Because of the blood loss of menstruation, women have higher dietary requirements for iron than males in order to prevent iron deficiency.

In most women, various unpleasant symptoms caused by the involved hormones and by cramping of the uterus precede or accompany menstruation. Such symptoms may include abdominal pain, migraine headaches, depression and irritability. Premenstrual stress syndrome (PMS or premenstrual syndrome), amenorrhea and dysmenorrhea can also occur. 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.

All of a woman's ova exist in the ovaries at the time of her birth; an estimated 250,000 to 400,000 immature eggs reside in each ovary. On average 450 of them develop into mature reproductive cells during a lifetime.


While the length of the first part of the cycle (from the onset of bleeding to ovulation) varies among women, the second part (from ovulation to onset of bleeding) almost always takes 14 days.

Sperm can survive for 3 to 4 days (possibly up to 7 days) inside a woman, so the most fertile period (the time with the highest likelihood of sexual intercourse leading to pregnancy) covers the time from some 5 days before ovulation until 1 to 2 days after ovulation. In a normal four-week cycle, this corresponds to the second and the beginning of the third week of the cycle. (Note that pregnancy can occur from intercourse at any time during the menstrual cycle, even during menstruation.) Various natural family planning 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.

Among women living closely together, the onsets of menstruation tend to synchronize somewhat. Researchers first described this phenomenon in 1971, and explained it by the action of pheromones in 1998 (Stern and McClintock 1998).

Hormonal control

Two main hormones each play a role in the control of the menstrual cycle: estrogen and progesterone. At the beginning of the menstrual cycle, the pituitary gland releases follicle stimulating hormone (FSH), signaling to the immature follicles to grow in the ovaries. (The follicle consists of a sac containing the egg.) Normally a woman's body will produce only one egg per period. Which gonad ovulates is essentially random; there is no left/right coordination involved. The first follicle to develop secretes inhibin, which shuts off the FSH, preventing more follicles from developing. Oestrogen levels rise as the developing follicle secretes that hormone. During ovulation, the follicle and the ovary's wall burst, releasing the egg; oestrogen levels peak at this time.

After ovulation the corpus luteum -- which develops from the burst follicle and remains in the ovary -- secretes both oestrogen and progesterone. Progesterone has the function of preparing the body for the possible pregnancy. If no pregnacy occurs, the corpus luteum dies and hormone levels fall, which causes the ejection of the endometrium with menstruation.

If pregnancy occurs, the placenta produces hormones to suspend the menstrual cycle — Chorionic gonadtrophin to maintain the corpus luteum, and Inhibin to prevent further ovulation — in addition to oestrogen and progesterone.

Oestrogen and progesterone are also the main ingredients of most birth control pills. Normally, a woman takes hormone pills for 21 days, followed by 7 days of non-functional placebo sugar pills; then the cycle starts again. During the 7 placebo days, a withdrawal bleeding occurs; this is not the same as an ordinary menstruation, and may be suppressed by skipping the placebos and continuing with the next batch of hormone pills. (Two main versions of the pill exist: monophasic and triphasic. With triphasic pills, skipping of the placebos can remove the pill's pregnancy protection.) In 2003, the United States FDA approved low-dose monophasic birth control pills which induce withdrawal bleedings only every 3 months.

Preference changes near ovulation Several studies have shown that women's preferences for men change during their most fertile days, before and shortly after ovulation. During this time, they will prefer different male scents, more masculine faces, and social presence and competitiveness in males considered as short-term partners. (Gangestad 2004) 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. (The synodical lunar month, the period between two new moons, lasts about 29 and a half days). Although some women's menstrual periods may conform to the lunar cycle, no necessary connection exists between lunar months and menstrual periods: humans show considerable variation in the lengths of their menstrual cycle, and the length of the menstrual cycle differs in different animals.

Cultural issues

During menstruation, women commonly use sanitary towels (worn outside the vagina) to prevent the soiling of clothes. Tampons (plugs made from absorbent material and inserted into the vagina) have become popular in Europe and in America, though not in Asia. A small but increasing number of women use reusable menstrual cups.


Mystics have sometimes elaborated "equivalencies", analogizing the waxing and waning of the moon with influences on human menstruation. In this spiritual, moon goddess, or astrological context some women call menstruation their "moontime".

Islam on menstruation

The Islamic world considers a woman "not in a state to have intercourse" during menstruation. A verse from the Holy Qur'an affirms this: "They ask you concerning menstruation. Say: that is an Adha (a harmful thing for a husband to have sexual intercourse with his wife while she is having her menses), therefore keep away from women during menses and go not unto them till they have purified (from menses and have taken a bath). And when they have purified themselves, then go in unto them as Allâh has ordained for you (go in unto them in any manner as long as it is in their vagina). Truly, Allâh loves those who turn unto Him in repentance and loves those who purify themselves (by taking a bath and cleaning and washing thoroughly their private parts, bodies, for their prayers, etc.)." (Al-Baqarah 2:222)

The parenthesised inserts come not from the Qur'an, but as interpolations by Dr. Muhammed Muhsin Khan, who based his interpretation on Islamic sources.

Dr. Muhiy al-Deen al-‘Alabi argues that scientific studies have disclosed some of the harm that is referred to in this verse: intercourse during menstruation leads to an increase in the flow of menstrual blood, because the veins of the uterus become congested and prone to rupture, and get damaged easily; and the wall of the vagina is also susceptible to injury, so the likelihood of inflammation increases, which leads to inflammation in the uterus and in the man’s penis, because of the irritation that occurs during intercourse. He further argues that having intercourse with a menstruating woman may be off-putting to both the man and his wife, because of the presence and smell of blood, which may make the man uninterested in sex.

Both Muslims and non-Muslims retain misconceptions on this issue. Incidentally, in Islam, anyone who bleeds for any reason is considered unclean and therefore exempt from ritual prayers until the blood discharge ceases.

Judaism on menstruation

A ritual exclusion applies to a woman while menstruating and for about a week thereafter, until she immerses herself in a mikvah (ritual bath).

Menstruation in other animals

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: no menstrual bleeding occurs.

External links

Harry Finley: Online museum of menstruation and women's health, Track your likely ovulatory date with this free Ovulation Calendar Free Software to watch the menstrual cycle etc. Leslie Botha-Williams, Women's Health Educator: A Woman's Guide to Understanding Her Hormone Cycle, An Islamic answer for the ruling of women menstruating


K. Stern and M. K. McClintock: "Regulation of ovulation by human pheromones.Gangestad et al.: "Women's preferences for male behavioral displays change across the menstrual cycle.

*Some information provided in whole or in part by