A uterine leiomyoma (uterine fibroid) is a benign tumor of myometrial (smooth muscle) origin. It is the most common solid benign uterine neoplasm. Fibroids are responsive to hormones (e.g stimulated by estrogens). They commonly accelerate in growth during pregnancy. 

A sub-serosal uterine leiomyoma is a sub type of uterine leiomyoma that often exophytically projects outwards from a sub-serosal location. A leiomyoma is often called sub-serosal if > 50 % of the fibroid protrudes out of the serosal surface of the uterus. They can be either sessile or pedunculated which if twisted will present as acute abdominal emergency.