Menstruation occurs in cycles. The ovaries prepare an egg (oocyte) for release and the womb (uterus) prepares a lining to nourish the egg if it is fertilised. When the egg is not fertilised, the lining of the womb is shed and a woman gets her period (menstrual bleeding)
when the thickened lining of the uterus endometrium leaves the body through the vagina. The lining of the uterus supplies a fertilised egg with nutrients, so if the egg isn't fertilised, then the lining is no longer needed.
