The uterus (womb) is a hollow, pear-shaped organ located in a woman's lower abdomen between the bladder and the rectum. The narrow, lower portion of the uterus is the cervix; the broader, upper part is the corpus. The corpus is made up of two layers of tissue.
In women of childbearing age, the inner layer of the uterus (endometrium) goes through a series of monthly changes known as the menstrual cycle. Each month, endometrial tissue grows and thickens in preparation to receive a fertilized egg. Menstruation occurs when this tissue is not used, disintegrates, and passes out through the vagina. The outer layer of the corpus (myometrium) is muscular tissue that expands during pregnancy to hold the growing fetus and contracts during labor to deliver the child.