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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 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.

The cycle

By convention, the onset of menstrual bleeding (menstruation or menses) marks the beginning of the cycle. Menstruation lasts for several days and is caused by the loss of the lining of the uterus. The uterus was ready to accept a fertilized ovum (egg), but none arrived, and so the lining (called endometrium) is expelled. Therefore, if menstrual bleeding occurs, a woman knows that she is not pregnant. During menstruation, women typically use tampons (plugs made from absorbent material and inserted into the vagina) or sanitary towels (worn outside the vagina) to prevent the soiling of clothes.

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 preceeded 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) can also occur.

All of a woman's eggs are present in the ovaries at the time of birth. An estimated 250,000 to 400,000 immature eggs exist in each ovary. Only a small fraction of them ever develop into mature eggs.

Timing

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

Sperm can live for 3-4 days (possibly up to 7 days) inside a woman, so the most fertile period (with the highest likelihood of sexual intercourse leading to pregnancy) is the time some 5 days before until 1-2 days after ovulation. In a normal four-week cycle, this corresponds to the second and the beginning of the third week of the cycle. (It is important to 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 fertile and infertile days in the cycle.

Hormonal control

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. Oestrogen levels rise as the hormone is secreted by the developing follicles. 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 developed from the burst follicle and remained in the ovary. The purpose of progesterone is to prepare the body for the possible pregnancy. Once the corpus luteum dies, hormone levels fall which causes the ejection of the endometrium with menstruation. Oestrogen and progesterone are also the main ingredients of most birth control pills.

Even though there are two ovaries, normally only one egg will be produced per period. Which ovary "wins" is essentially random; there is no left/right coordination involved. As the level of the follicle stimulating hormone (FSH) increases, it stimulates the production of a follicle, the follicle secretes inhibin, which shuts off the FSH, preventing more follicles from developing. So each month, it depends on whether the left or right ovary is the lucky one to produce the first follicle.

Terminology, and menstruation in other animals

The terms "menstruation" and "menses" come from the Latin mensis (month) from Greek mene (moon) in reference to the fact that the lunar month is also approximately 28 days long. There is no connection between lunar months and menstrual periods however, as is shown by the fact that the great apes have menstrual cycles that vary from 29 days in orangutans to 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.