If fertilization does not take place and no cluster of egg cells is implanted in the uterus, the production of hormones declines and the blood-filled endometrium, no longer needed to nourish a possible embryo, breaks up. Over several days it is expelled from the uterus, sometimes with the help of contractions that may range from mild to painful. This shedding of tissue and blood is called menstruation, or a monthly period. The monthly sequence of events is called the menstrual cycle and its sole purpose is to prepare your body for a probable pregnancy.

This cycle of events occurs every month unless an egg is fertilized and successfully attaches to the uterus lining. When such an attachment takes place, the lining is not expelled, there is no menstrual bleeding, and a pregnancy is under way.