The uterus is a hollow, muscular organ located in the female reproductive system. It plays a crucial role in pregnancy, as it is where a fertilized egg implants and develops into a fetus. The uterus has three main parts: the fundus (the top), the body (the main part), and the cervix (the lower part that opens into the vagina). During the menstrual cycle, the uterus undergoes changes in its lining, known as the endometrium. If pregnancy does not occur, this lining is shed during menstruation. The uterus is also involved in childbirth, contracting to help deliver the baby through the vagina.