The exact cause of uterine fibroids is not known. Fibroids begin when cells overgrow in the muscular wall of the uterus.

After a fibroid develops, the hormones estrogen and progesterone appear to influence its growth.¹ A woman's body produces the highest levels of these hormones during her childbearing years. After menopause, when hormone levels decline, fibroids usually shrink or disappear.