Loss of blood from the UTERUS via the VAGINA occurring when the women's level of oestrogen hormones (OESTROGENS), PROGESTERONE hormone or PROGESTOGEN drugs falls quickly. The withdrawal bleeding that happens at the end of each month's cycle of combined oral contraceptive pills (see CONTRACEPTION) imitates the woman's menstrual period (see MENSTRUATION) but is normally briefer and less in amount.