What are the causes of uterine fibroids?

The exact cause of uterine fibroids is not known, but it may be related to changes in the levels of the hormones estrogen and progesterone and in proteins called growth factors.

For example, pregnancy, use of birth control pills, or hormone therapy may increase the growth of fibroids.

It also appears that women may inherit the tendency to develop fibroids.

Fibroids rarely occur in women younger than 20. They occur most frequently in women in their childbearing years. When a woman goes through menopause, these tumors usually shrink.

Uterine fibroids often grow during pregnancy and they degenerate after menopause. From these observations and certain studies researchers are fairly certain that the female hormones, both estrogen and progesterone, play a role in their growth. Their role, however, is not clear. Some theories about the relationship to fibroids and estrogen include the following:

Estrogen patterns in fibroids are similar to those in pregnancy. That is, like smooth muscle cells in the uterus during pregnancy, fibroid cells exposed to female hormones do not respond normally to signals that would make them self-destruct and return to a nonpregnant state. (This natural self-destruction is a process called apoptosis). Instead, they continue to grow.

Some evidence suggests that estrogen may inhibit a tumor-suppressor gene called p53 in fibroid tissue, therefore triggering cell proliferation leading to fibroid growth. (P53 plays a role in some cancer-cell growth, although in this case the process is not malignant.)

Inherited genetic factors may be important in many cases of fibroids. Researchers are investigating unique genetic factors that regulate hormones. Proteins called growth factors may be responsible for some of the abnormalities leading to uterine muscle overgrowth and fibroids.

Scientists have identified chromosomes carrying a total of 145 genes that may have an effect on fibroid growth. Some experts report that uterine fibroids are inherited from paternal genes (the father's side).