The menstrual cycle is a term used to describe the sequence of events that occur within a women’s body as it prepares for the possibility of pregnancy each month. A menstrual cycle is considered to begin on the first day of a period. The average cycle is 28 days long; however, a cycle can range in length from 21 days to about 35 days. 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 egg (ovum), but none arrived, and so the lining (called endometrium) is expelled. 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 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, which is about 7–14 cm long, 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. This blood loss is the reason that women have higher dietary requirements for iron than males and need to prevention iron deficiency.
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 during a lifetime.