Menstruation is one of the signs of puberty. The purpose of menstruation is to prepare the women's bodies for pregnancy and childbirth. Every month, the lining of the uterus thickens to receive and nourish a fertilized egg (if a woman becomes pregnant). If, however, the egg does not become fertilized, the uterus lining is shed. This process of shedding the lining is called menstruation.

A woman's first menstrual period generally occurs between the ages of 9 and 16. The first period is called menarche. There is no way to predict when a girl will have her first period; the age at which her mother started menstruating can give a rough estimate. There is a theory that when the amount fat in the girl's body reaches a certain level, it gives the body the permission to proceed menstruating. Women who lose a lot of their body fat through athletic training may...