Most women begin to menstruate between 11 and 14 years of age and continue until they reach menopause some 40 years later. Although the “normal” cycle is 28 days, there is no cause for concern if periods are spaced 25 to 34 days apart, since precise regularity is rare. During the “typical” 3-to-5 day menstrual period, the average woman loses less than 2 ounces of blood.

The first menstrual period separates childhood from adolescence. Along with breast enlargement and the growth of pubic hair, it signals a young woman's sexual maturity. This monthly vaginal discharge of blood, secretions, and cells from the surface of the uterus is the final step in a complex cycle that prepares the body to conceive a child.

Each cycle begins when, responding to a cascade of hormones, a dormant egg cell within one of the ovaries begins to ripen. Cells around the maturing egg release the female hormone estrogen, prompting the lining of the uterus (the endometrium) to thicken in preparation for receipt of a fertilized egg.

When it reaches maturity, the developing egg bursts from the ovary and begins its trip down the fallopian tube to the uterus in a process called ovulation. The supporting cells left behind after ovulation then begin to manufacture another hormone, progesterone, in addition to estrogen. This second hormone fosters further growth in the lining of the uterus.

If fertilization does not take place, the ovum dies and production of estrogen and progesterone stops. Robbed of its sustaining hormones, the thickened lining of the uterus begins to break down. The dead endometrial cells, along with a little blood, are then discharged in the menstrual flow.

Normal menstruation depends on the delicate orchestration of the hormones that govern development of the egg. The menstrual cycle can also be affected by disease, diet, emotions, and defective development of the reproductive organs.