We do not know exactly why women develop these tumors. Genetic abnormalities, alterations in growth factor (proteins formed in the body that direct the rate and extent of cell proliferation) expression, abnormalities in the vascular (blood vessel) system, and tissue response to injury have all been suggested to play a role in the development of fibroids.

Family history is a key factor, since there is often a history of fibroids developing in women of the same family. Race also appears to play a role. Women of African descent are two to three times more likely to develop fibroids than women of other races. Women of African ancestry also develop fibroids at a younger age and may have symptoms from fibroids in their 20s, in contrast to Caucasian women with fibroids, in whom symptoms typically occur during the 30s and 40s. Early pregnancy decreases the likelihood that fibroids will develop. Fibroids have not been observed in girls who have not reached puberty, but adolescent girls may rarely develop fibroids. Other factors that researchers have associated with an increased risk of developing fibroids include having the first menstrual period (menarche) prior to age 10, consumption of alcohol (particularly beer), uterine infections, and elevated blood pressure (hypertension).

Estrogen tends to stimulate the growth of fibroids in many cases. During the first trimester of pregnancy, about a third of fibroids will enlarge and then shrink after the birth. In general, fibroids tend to shrink after menopause, but postmenopausal hormone therapy may cause symptoms to persist.

Overall, these tumors are fairly common and occur in about 70% to 80% of all women by the time they reach age 50. Most of the time, uterine fibroids do not cause symptoms or problems, and a woman with a fibroid is usually unaware of its presence.