Menstruation is one part of a woman's cycle when the lining of the uterus (endometrium) is shed. This occurs throughout a woman's reproductive life. With each monthly cycle, the endometrium prepares itself to nourish a fetus. Increased levels of estrogen and progesterone help thicken its walls. If fertilization does not occur, the endometrium, along with blood and mucus from the vagina and cervix make up the menstrual flow that leaves the body through the vagina during the period.