What causes fibroids?

Smooth muscle tumor: Fibroids are growths of the smooth muscle of the uterine wall. These are almost always benign, but they can become large. Different people may have 1-2 large fibroids or multiple smaller fibroids, or a combination. Most are asymptomatic, but they can cause abnormal bleeding, pain, infertility, or mass effect (eg, pressing on the bladder affecting bladder function).

Not known...: ...., but genetic factors and hormone status plays a part.

Not known: It is not known what causes fibroids.