Menstruation is the flow of blood from a woman's uterus (womb) about every 28 days. At ovulation (release of the egg from the ovaries) the uterus prepares itself for a fertilised egg by creating a blood lining to nourish a developing baby. If the egg is not fertilised this lining is not needed so it is released through the vagina as a menstrual period. Menstruation usually starts in adolescence (age 14-18) and ends at the menopause (age 45-55), but everyone is different.
Attribution: Straight Talk Foundation