Menstruation, the loss of blood and cells that occurs about once a month in most women of child-bearing age, takes place through the vagina. During each month, blood and cells build up in the lining of a woman's uterus (or womb), a hollow, pear- shaped organ that holds a baby during pregnancy. The thickening of the lining prepares the uterus for pregnancy. If pregnancy does not occur, the lining breaks down. The blood and cells are discharged through the vagina, which is a canal that leads from the uterus to the outside of the body. The process of menstruation lasts from three to seven days, and this period of time is called the menstrual period.