The uterus is the hollow, thick-walled muscular organ where implantation of the fertilised egg and foetal development occur. It is positioned in the pelvic cavity anterior to the rectum and posterior to the urinary bladder. The uterine (Fallopian) tubes open into its superior angles. The cervix is the lower, narrow portion of the uterus; the vagina is the birth canal; the endometrium is the inner lining of the uterus.