Uterine fibroids are benign tumors that originate in the uterus (womb). Although they are composed of the same smooth muscle fibers as the uterine wall (myometrium), they are much denser than normal myometrium. Uterine fibroids are usually round.Uterine fibroids are often described based upon their location within the uterus. Subserosal fibroids are located beneath the serosa (the lining membrane on the outside of the uterus). These often appear localized on the outside surface of the uterus or may be attached to the outside surface by a pedicle. Submucosal (submucous) fibroids are located inside the uterine cavity beneath the inner lining of the uterus. Intramural fibroids are located within the muscular wall of the uterus.