A period, or menstruation, marks the beginning of the process by which the uterus, or womb, prepares itself for pregnancy. Each month the lining of the uterus, called the endometrium, is shed from the body, producing a period, and a new lining is grown to replace it. Whilst it's convenient to think of the menstrual cycle as a monthly event, indeed we get the word menstruation from the 28 day cycle of the moon, only about 12% of women actually have a period every 28 days.
