The lining of the womb, or uterus, starts to thicken. About 2 weeks later, one of the ovaries releases an egg, or ovum. If sperm from a man's body does not join with the egg, that is, it is not fertilized, then the thick lining of the uterus is not needed. In about another two weeks, her body gets rid of this lining through the vagina. This is called having a period, or menstruation.
Many women are uncomfortable or have cramps during their periods.
Exercise, a heating pad or hot water bottle, and a pain reliever can help. If these ideas don't work, ask a doctor or school nurse for help. Periods usually last between 3 and 7 days. Many things can affect your period, such as stress or sickness or fast weight loss. After a while, most women find that their periods become regular.