Uterine fibroids are the most common pelvic tumor. Fibroids may be seen as many as 1 in every 5 women in their childbearing years (the time after starting menstruation for the first time and before menopause.)
Fibroids usually affect women over age 30. They are rare in women under 20 or in those who have gone through menopause. They are more common in African-Americans than Caucasians.
The cause of uterine fibroid tumors is unknown. However, fibroid growth seems to depend on the hormone estrogen. As long as a woman with fibroids is menstruating, a fibroid will probably continue to grow, usually slowly.
Fibroids can be so tiny that you need a microscope to see them. However, they can also grow very large. They may fill the entire uterus, and may weigh several pounds. Although it is possible for just one fibroid to develop, usually there is more than one.
Sometimes, a fibroid hangs from a long stalk, which is attached to the outside of the uterus. This is called a pedunculated fibroid.
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 affect fibroid growth. Some experts report that uterine fibroids are inherited from paternal (the father's) genes.
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 cancerous.)
The formation of fibroids may be attributable to abnormalities in substances called growth factors. These special proteins, secreted by different cell types, are responsible for cell-to-cell interaction. Many of these substances regulate a process called angiogenesis, which causes new blood vessels to sprout from pre-existing ones. The production of new blood vessels then feeds any existing growth, such as a fibroid.
The growth factors that appear to play an important role in many female reproductive disorders are Basic Fibroblast Growth Factor (BFGF) and Vascular Endothelial Growth Factor (VEGF). BFGFs are involved in the proliferation of cells that form connective tissue, which supports the body's organs and structure. VEGFs are involved with cell growth in smooth muscles that line blood vessels. Some evidence suggests they play a role in uterine fibroids.
Other growth factors being studied specifically for fibroids include Insulin-like Growth Factor (IGF)-I, Epidermal Growth Factor (EGF), Platelet Derived Growth Factor (PDGF), and Transforming Growth Factor (TGF).