It has been recognized for a long time that fibroids occur in women with high ovarian function; they never occur before menarche; they stop growing after menopause and gradually degenerate and shrink; they accelerate during pregnancy; that is to say, the growth of fibroids is closely related to high estrogen levels. Interestingly, women who used to be sports enthusiasts had a lower frequency of fibroids later in life. The risk of fibroids is reduced in women who smoke, especially in women who have recently smoked, and the risk of fibroids is reduced by 40%, independent of the amount smoked. Genetic factors have an important influence on the development of fibroids. The family risk of fibroids in sisters with fibroids is 26.6%, and the risk of fibroids in daughters of those with fibroids is 19.7%.