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Uterine Fibroids: Overview and Risk Factors

Uterine leiomyomas, or fibroids, are benign tumors of the uterus composed of smooth muscle and connective tissue. Fibroids are very common, present in at least one–quarter of women by the age of 40.

Fibroids are classified by anatomic location as intramural (within the myometrium), submucosal (underlying the endometrium), or subserosal (underlying the uterine serosa). There is no identifiable cause of uterine fibroids. However, estrogen is necessary for their growth, as many grow during pregnancy and then recede at menopause. Further, higher parity and oral contraceptive use have been shown to decrease the risk of fibroid formation.

Most uterine fibroid cases are asymptomatic. However, symptoms may include uterine bleeding, resulting in prolonged or heavy menstrual flow and possibly anemia; dysmenorrhea; urinary frequency and urgency; constipation, dyspareunia; and abdominal tenderness. Complications of pregnancy are more common in women with fibroids, including miscarriage, placental abruption, and premature labor.

Risk Factors

African American women are up to 3 times more likely to have fibroids compared with white women, and often have more severe disease at a younger age.1

Age. Fibroids occur during the reproductive years, most commonly becoming clinically apparent during the fourth and fifth decades of life. They do not occur in prepubescent girls and usually shrink at menopause.

Genetics. Monozygotic twins have a 2 to 3 times greater risk of fibroids than dizygotic twins when one twin is affected.2

Pregnancy. Parity appears to decrease the risk of fibroids.

Oral contraceptive pills. Although these appear to be protective, the Nurses’ Health Study showed an increased risk in women who used oral contraceptive pills at ages 13 to 16. Low–dose oral contraceptives and menopausal hormone therapy are not contraindicated in women with fibroids.

Some evidence suggests that cigarette smoking may decrease the risk of fibroids. Of course, the health risks of smoking far outweigh this potential benefit.


Uterine Fibroids: Diagnosis >>