A fibroid is the most common tumor (abnormal mass of tissue) found in the pelvis. Such a tumor develops most often between the ages of 35 and 45 years, seldom before age 20. Fibroids do not occur at all before puberty. After a woman completes menopause, they generally stop growing and may even disappear.

Usually a fibroid is found in the uterus where it is the most common form of uterine mass. A fibroid can also develop on another structure that contains smooth muscle cells. In very rare cases, it can even "invade" another organ when it grows too large to confine itself to its original location.

A fibroid is a solid tumor that contains mostly smooth muscle held together by fibrous tissue. (That's how it came by its popular name.) Other names for fibroids are leiomyomas, myomas, fibromas, and fibromyomas.

Fibroids usually occur as multiple tumors that tend to grow very slowly. Sometimes, however, a woman may have a single fibroid the size of a grapefruit or even one so large that it fills the entire abdomen. On the other hand, a fibroid tumor may be so small that it can be seen only under a microscope. It can weigh as much as 25 or 50 pounds, with the largest fibroid tumor ever reported weighing more than 140 pounds.

It is difficult to know just how many women have fibroids, since unless they cause a problem, a woman may never realize she has them. It is, however, estimated that fibroids occur in about 30 percent of women aged 25 to 45. They are more common in women who are overweight. Your risk is lower if you have had a baby or if you smoke.

For some reason, black women seem more likely to develop fibroids than women in other racial groups. Almost half of all black women more than 30 years of age have fibroids compared with 20 percent of white women of the same age group. Interestingly, studies have found a low incidence of fibroids among women in Africa. Overall, however, fibroids tend to be larger and more numerous in black women at diagnosis.