What causes uterine fibroids, how common are they, and what happens if they are left untreated?

Multifactorial: Fibroids are non-cancerous muscle tumors of the uterus. They develop because of a mutation in a uterine muscle cell that causes that cell to lose control of its growth. They are very common in some ethnic groups, and present in women of all backgrounds. They can cause heavy bleeding, pain, infertility, and pregnancy complications, or may cause no problems at all, depending on where they are.

For most nothing: 1 in 3 women of childbearing age have fibroids and up to 80% of african-american women have them. For most women the fibroids will be asymptomatic and no treatment is necessary. For some, even though they aren't causing symptoms, they may cause infertility.