Uterus: A hollow, pear-shaped organ that is located in a woman's lower abdomen, between the bladder and the rectum. The narrow lower portion of the uterus is the cervix (the neck of the uterus). The broader upper part is the corpus, which is made up of three layers of tissue. In women of childbearing age, the inner layer (endometrium) of the uterus 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 middle layer (myometrium) of the uterus is muscular tissue that expands during pregnancy to hold the growing fetus and contracts during labor to deliver the child. The outer layer (parametrium) also expands during pregnancy and contracts thereafter.