Fibroids are benign (non-cancerous) tumors that sometimes develop inside a woman’s uterus. The uterus is the hollow, muscular organ in a woman’s pelvis where a fertilized egg implants and grows during pregnancy.

Uterine fibroids are also called fibromyomas, leiomyomas or myomas. Fibroids form from the smooth muscle cells that line the wall of the uterus (myometrium). Sometimes, a single cell reproduces repeatedly until a pale, firm, rubbery growth develops that is different from normal uterine tissue.

A single fibroid can develop or multiple fibroids may form. They range in size from tiny, undetectable growths to bulky masses that can expand and distort the uterus, and rarely, the cervix.

These non-cancerous tumors commonly appear during a woman’s reproductive years and tend to grow slowly over time. Why fibroids develop is not completely understood, but it may be related to increased levels of the hormones estrogen and progesterone. For instance, fibroids tend to increase in size faster during pregnancy, when levels of estrogen increase. Uterine fibroids typically shrink or disappear after the cessation of menstruation (menopause).

According to the American Society for Reproductive Medicine (ASRM), about one out of every four women in the United States develops uterine fibroids. They occur more frequently in African Americans than Caucasians. Fibroids are rare before age 20 but can affect up to 40 percent of women over age 30, according to the National Institutes of Health.

In many cases, women are not aware that they have uterine fibroids because fibroids often do not cause symptoms. In such cases, treatment is not necessary. However, if the fibroids grow too large and/or the location compromises the health of the uterus and nearby organs, severe symptoms can develop and require medical attention.

Rarely, fibroids may interfere with a woman’s ability to conceive (infertility). Sometimes the changes that are taking place in the endometrium as a result of the fibroids may prevent the implanting of a fertilized egg to the uterine wall. Other times, the fibroids may compress or block the fallopian tubes, thereby preventing sperm from fertilizing an egg. In some cases, a large fibroid may interfere with the growth of a fetus and contribute to a miscarriage.

Although uterine fibroids usually do not cause infertility, the condition can be associated with pregnancy complications including placenta previa and early labor and premature delivery. In such cases, complications are often due to large fibroids occupying needed space in the uterus. Pregnant women with fibroids also may require a Caesarean delivery in more cases, especially if the fibroids are blocking the birth canal or causing the baby to be positioned improperly (e.g., breech).

In addition, there is a very small possibility that some fibroids may become cancerous. According to the ASRM, the risk of fibroids becoming cancerous (malignant) is about one in 1,000 in women of reproductive age. However, after menopause a fibroid that suddenly becomes larger may be cancerous (leiomyosarcoma).