The cause of fibroid tumors of the uterus is unknown. However, it is suggested that fibroids may enlarge with estrogen therapy (such as oral contraceptives) or with pregnancy.

Their growth seems to depend on regular estrogen stimulation, showing up only rarely before the age of 20 and shrinking after menopause. As long as a woman with fibroids is menstruating, the fibroids will probably continue to grow, although growth is usually quite slow.

Fibroids can be microscopic, but they can also grow to fill the uterine cavity, and may weigh several pounds. Uterine fibroids are the most common pelvic tumor and they may be present in 15 to 20% of reproductive-age women, and 30 to 40% of women over 30.

Fibroids occur 3 to 9 times more frequently in African-American women than in Caucasian women.

It is possible for a single fibroid to develop although normally there are a number of them, which begin as small seedlings spread throughout the muscular walls of the uterus.

They slowly enlarge and become more nodular, frequently intruding into the cavity of the uterus or growing out beyond the normal boundary of the uterus. Rarely, a fibroid will hang from a long stalk attached to the outside of the uterus. This is called a pedunculated fibroid.