Menstruation is often called having a "period." A period is the 2–7 days that a girl or woman has her menstrual flow, which is when blood and tissue leave her body through her vagina.
Why does this happen? Each month, blood and tissue build up in the uterus to prepare for a fertilized egg in case a woman becomes pregnant. (The uterus is the place inside a woman's body where a baby will grow.) If the egg isn't fertilized, that lining leaves the body through the vagina and the girl or woman has her period.