NEW YORK (Reuters Health) - Exercising regularly may help prevent women from developing uterine fibroids, a new study shows.

Fibroids, also referred to as uterine leiomyomas, are benign tumors that grow in the uterus and can cause infertility, bleeding, pain and pregnancy difficulties. They are the leading reason for hysterectomies among US women, and are particularly common among African-Americans, Dr. Donna Day Baird of the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina and colleagues note in the American Journal of Epidemiology.

Little is known about how fibroids might be prevented, Baird and her team add, but hormones appear to trigger the tumors' onset and growth. Research has suggested that exercise can help prevent other hormone-related conditions, such as breast and endometrial cancer. They therefore determined if exercise might have any effect.

The researchers evaluated 734 African-American women and 455 white women between 35 and 49 who belonged to a Washington, DC-based health plan. The researchers screened the women for fibroids using ultrasound and reviewed medical records to see if the women who underwent hysterectomies had a history of fibroids.

The more active the women were, the less likely they were to have fibroids of any size, Baird and her team found. Those who exercised for 7 hours or more weekly had a 40 percent lower fibroid risk than those who exercised for less than 2 hours a week. Women who reported at least 4 hours of vigorous exercise weekly were less likely to develop tumors than those who exercised less.

The relationship between exercise and tumor onset was stronger than the link between exercise and tumor growth, and was seen for both black and white women.

When the researchers removed women with severe fibroids that might have interfered with physical activity from their analysis, the association between exercise and lower fibroid risk remained.

"Women have little control of other factors known to affect fibroids... but exercise is a modifiable practice and also has other positive effects on women's health," Baird and her team conclude.