Every month during a woman's fertile years, her body goes through a natural process called menstruation (MEN-stru-WAY-shun). Menstruation is the process by which the lining of the uterus (the endometrium) is shed as menstrual bleeding. This process is an intricate one, controlled by the brain and a complex hormone-signaling system. Menstruation usually happens once a month, except during pregnancy.

Every time a woman gets her period, it marks the beginning of a new cycle. A menstrual cycle lasts, on average, 28 days, but this can vary. The word menstruation comes from the Latin word menses, which means month. A menstrual cycle that lasts anywhere from 25 to 35 days is considered "normal."

Although menstrual cycles can vary in length, the number of days between ovulation and the menstrual period is consistent, approximately 14 days (11-16 is the normal variation). For example, if a woman's typical cycle length is 31 days, then the first half of the cycle is 16 days and ovulation occurs on the 17th day.