Menstruation, or period, is a woman's monthly bleeding. Every month, your body prepares for pregnancy. If no pregnancy occurs, the uterus sheds its lining. The menstrual blood is partly blood and partly tissue from inside the uterus, or womb. It passes out of the body through the vagina. Periods usually start around age 12 and continue until menopause, at about age 51.
MedlinePlus - Menstruation