Uterine fibroids are common noncancerous tumors of the muscular wall of the uterus, affecting as many as eight in 10 women by the age of 50. Fibroids more commonly affect women over age 30. They are also more common in African-Americans than Caucasians. Overweight and obese women also have a greater risk of developing fibroids. Hormonal and genetic factors contribute to their growth.