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

Causes and risk factors

The cause of uterine fibroids is unknown. However, their growth has been linked to 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 also grow very large. They can 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

Incidence

Uterine fibroids are common. 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. Half of all women have fibroids by age 50. Fibroids are rare in women under age 20. They are more common in African-Americans than Caucasians.

Note: There are often no symptoms. Your health care provider may find them during a physical exam or other test. Fibroids often shrink and do not cause symptoms in women who have gone through menopause.

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