The organ of female mammals in which the embryo develops. Paired in most mammals but single in humans, it is situated between the bladder and rectum and is connected to the fallopian tubes and to the vagina. The lining (see endometrium) shows cyclical changes (see menstrual cycle; oestrous cycle) associated with egg production (see ovarian cycle) and provides a thick spongy layer in which the fertilized egg becomes embedded. The outer wall of the uterus (myometrium) is thick and muscular; by contracting, it forces the fully grown fetus through the vagina to the outside.