When a woman menstruates, her body discards the lining of the uterus. The blood flows from the uterus through a tiny opening, passing out the blood through the vagina. This is called menstruation.
A menstrual or period cycle is calculated from the first day of the period to the first day of the next period. An average menstrual cycle is 28 days long, but some women may have longer or shorter cycles. It can range from 21 to 35 days in adults and 21 to 45 days in teens.