Uterine fibroids are noncancerous (benign) tumors that develop in the uterus (womb), a female reproductive organ.

Uterine fibroids are the most common pelvic tumor. As many as 1 in 5 women may have fibroids during their childbearing years (the time after starting menstruation for the first time and before menopause).

Fibroids usually affect women over age 30. They are rare in women under 20, and often shrink and cause no symptoms in women who have gone through menopause. They are more common in African Americans than Caucasians.

The cause of uterine fibroid tumors is unknown. However, fibroid growth seems to depend on the hormone estrogen. As long as a woman with fibroids is menstruating, a fibroid will probably continue to grow, usually slowly.

Fibroids can be so tiny that you need a microscope to see them. However, they can grow very large. They may fill the entire uterus, and may weigh several pounds. Although it is possible for just one fibroid to develop, usually there are more than one.

Fibroids are often described by their location in the uterus:

• Myometrial -- in the muscle wall of the uterus
• Submucosal -- just under the surface of the uterine lining
• Subserosal -- just under the outside covering of the uterus
• Pendunculated -- occurring on a long stalk on the outside of the uterus or inside the cavity of the uterus