The uterus is an organ in the female reproductive system that is responsible for menstruation, fertility, and pregnancy. It is a hollow, muscular, pear-shaped organ that sits between the rectum and bladder in the pelvis. The uterus has several sections, including the fundus, corpus, isthmus, and cervix. The fundus is the broad curved upper area where the fallopian tubes connect to the uterus, while the cervix is the lower, narrow neck region that extends downward from the isthmus until it opens into the vagina. The uterus has three layers: the endometrium, which is the inner lining that thickens during the menstrual cycle and sheds during menstruation; the myometrium, which is the thick middle muscle layer that expands during pregnancy to hold the growing baby and contracts during labor to push the baby out; and the serosa, which is the smooth outer layer that covers the uterus and makes it easy for the uterus to slide and move within the pelvis as needed. The average dimensions of the uterus in an adult female are 8 cm long, 5 cm across, and 4 cm thick with a volume of 80 to 200 mL. The uterus is supported by the pelvic floor muscles and perineal body, as well as ligaments in the pelvis, lower back, and hips.