What Are They?

Uterine fibroids are benign (noncancerous) tissue growths inside the uterus. These fibroids are very common, and they can develop at various stages of life. Uterine fibroids (leiomyomas or myomas) often become detectable by gynecological examination during women’s reproductive years. It is extremely rare for cancer to develop from a fibroid of this type. Therefore, these fibroids are generally not associated with risk of cancer.

The fibroids are categorized by their location inside, or in the vicinity of, the uterus. Those that bulge and protrude inside the cavity of the uterus are classified as submucosal fibroids. Those growing in the muscular wall of the uterus are known as intramural fibroids. And those which project outside the uterus are classed as subserosal fibroids.

Many fibroids are so small that they are not visible to the human eye. Others grow in size enough to enlarge and distort the shape of the uterus. A woman may have just one fibroid or many. In extreme cases, multiple fibroids expand the uterus to the extent that it touches the ribs.

Causes

The prevailing medical opinion is that a fibroid is formed when a single stem cell repeatedly divides in the smooth uterine muscular tissue of the uterus (the myometrium), until a firm, rubbery mass is formed and is distinguishable from nearby non-fibroid tissue.

The cause(s) of these uterine fibroids remain(s) unknown. However, clinical experience and research indicate that the following factors appear to be associated with such fibroids:

• Hormones — During each menstrual cycle, the two female hormones, estrogen and progesterone, stimulate development throughout the uterine lining, in preparation of the uterus for pregnancy. This natural process apparently promotes fibroid growth. The fibroids contain greater amounts of estrogen and progesterone receptors than do the normal muscle cells of the uterus. Alternatively, the decreases in these hormones during menopause appears to account for the tendency of fibroids to shrink in size after menopause.
• Non-hormonal growth factors — Fibroid growth may also be affected by other growth factors, such as insulin-like substances that assist the body in maintaining tissue.
• Genetic changes — Many fibroids are found to contain changes in genes which are unlike those in normal muscle cells of the uterus.

Patterns of uterine fibroid growth vary. Fibroids may develop rapidly or slowly. Or, they may discontinue growing and stay the same size. Growth of some fibroids may stop, start, and stop again. Others may shrink in size. In many cases, fibroids that are detectable during pregnancy shrink, or entirely disappear as the uterus returns to a normal size after pregnancy.

Risk Factors

Other than being a woman of childbearing age, few risk factors are known to be associated with uterine fibroids. These few factors which may have potential effects on development of fibroids include:

• Environmental factors — using birth control, beginning menstruation at an early age, drinking alcohol, Vitamin D deficiency, obesity, and/or maintaining a diet higher than recommended in red meat and lower than recommended in green vegetables may place you at increased risk for fibroid development.
• Race — African American women are at higher risk for developing fibroids than women of other racial origins. Additionally, African American women are more likely to develop fibroids at earlier ages, and are at higher risk of developing larger fibroids or greater numbers of fibroids.
• Heredity — Your risk of developing uterine fibroids is higher if your sister or mother has had fibroids.

Complications

Typically, uterine fibroids are not dangerous. However, these can be a cause of discomfort. Additionally, fibroids may result in complications, including anemia, in cases of significant blood loss.