Uterine fibroids are benign (noncancerous) growths that develop in the wall of the uterus. They are usually round or semi-rounded in shape and vary in size from very small to large. The main cause of uterine fibroids is unknown, although certain factors, such as genetics and hormones, are thought to contribute to their development. Factors such as age, race, and family history may also play a role. Some research suggests that an imbalance of the hormones estrogen and progesterone may contribute to the growth of fibroids. Other potential causes include environmental toxins, obesity, and chronic inflammation.