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. 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.
Uterine fibroids (leiomyomata) are noncancerous growths that develop in or just outside a woman’s uterus (womb). Uterine fibroids develop from normal uterus muscle cells that start growing abnormally. As the cells grow, they form a benign tumor.