The uterus, also known as the womb, is a muscular, pear-shaped organ located between a woman’s rectum and her bladder. During pregnancy, this is where the fetus develops. In women of child-bearing age, the inner layer of the uterus (endometrium) thickens in preparation for the implantation of a fertilized egg each month. If there is no fertilized egg, the endometrium is shed through the vagina. This process is called menstruation.