The menstrual cycle is the periodic change in a woman's body that occurs every month between puberty and menopause and is related to reproduction. The average human menstrual cycle is 28 days long, but it can range from 21 to 35 days. It is controlled by hormones.
Menstruation, commonly called a woman's period, is the approximately monthly shedding of the uterine lining; a sign that conception has not taken place and the blood and tissues that have built up throughout the month preparing for fertilization leave the body through the vagina. Menstruation is a normal, natural cyclic process occurring in all healthy adult women between puberty and menopause. Young women can begin to menstruate at any time between the ages of eight and eighteen; mature women might experience menopause sometime between the ages of forty and sixty. Women may have their period every eighteen days to thirty-six days. They may bleed some for two or three days or seven or eight – all of which are normal.
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 egg is released by the ovary and enters the fallopian tube. In some women, ovulation is accompanied by 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 takes about one day, and the egg is available to be fertilized during this period.
In the meantime, the endometrium has started to grow again. If fertilization occurs, the egg implants in the wall of the uterus and major changes take place, with the menstrual cycle being suspended for the length
of the pregnancy. If no fertilization occurs, the endometrium is lost with bleeding and the cycle starts again.
About 50 millilitres of blood are lost during menstruation. The blood is prevented from clotting by an enzyme called plasmin contained in the endometrium. In most women, menstruation is preceded or accompanied by various unpleasant symptoms caused by the involved hormones and by cramping of the uterus. Among these are abdominal pain, migraine headache, 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 are present in the ovaries at the time of birth. An estimated 250,000 to 400,000 immature eggs reside in each ovary. On average 450 of them develop into mature reproductive cells.
A somewhat common error of people who have heard about the menstrual cycle and ovulation is to assume, for contraceptive purposes, that cycles are always regular and that ovulation always occurs 14 days after the end of the menses. This may lead to unintended pregnancies.
Among women living closely together, the onsets of menstruation tend to synchronize somewhat. This was first described in 1971 and explained by the action of pheromones in 1998.
The main hormones involved in control of the menstrual cycle are oestrogen and progesterone. At the beginning of the menstrual cycle, the pituitary gland releases follicle stimulating hormone (FSH), signaling the immature follicles to grow in the ovaries. The follicle is a sac containing the egg. Normally only one egg will be produced 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 hormone is secreted by the developing follicle. During ovulation, the follicle and the ovary's wall burst, releasing the egg; oestrogen levels are maximal.
After ovulation, both oestrogen and progesterone are secreted by the corpus luteum which develops from the burst follicle and remains in the ovary. The purpose of progesterone is to prepare 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.
The terms "menstruation" and "menses" come from the Latinmensis (month) from Greekmene (moon) in reference to the fact that the lunar month is also approximately 28 days long. Although some women's menstrual periods may conform to the lunar cycle, there is no necessary connection between lunar months and menstrual periods: humans show considerable variation in the lengths of the menstrual cycle, and the great apes have menstrual cycles that vary from an average of 29 days in orangutans to an average of 37 days in chimpanzees.
A regular menstrual cycle as described here only occurs in the higher primates. Females of other mammalian species go through certain episodes called "estrus" or "heat" in each breeding season. During these times, ovulation occurs and females are receptive to mating, a fact advertised to males in some way. If no fertilization takes place, the endometrium is reabsorbed by the uterus and no menstrual bleeding occurs.
