Menstrual cycle
This stage is the start of the menstrual cycle, and is counted as day one of the cycle.  This stage starts when the amount of progesterone and estrogen in a woman's blood drop.  This causes the endometrium - the thick lining of the uterus - to leave the woman's body through menstruation.  It will appear as bleeding from the vagina.  This stage lasts from 4–6 days.