Uterine fibroids cost the U.S. billions each year in lost work days, pregnancy problems and treatments, a new study finds.

Uterine fibroids are common, non-cancerous growths that form from muscle cells and other tissue in the wall of the uterus. In the U.S., it's estimated that up to 70 percent of white women develop fibroids at some point by age 50; among African-American women, that number is closer to 80 percent.

Often, fibroids cause no symptoms. But at least one-quarter of U.S. women have fibroid symptoms like heavy menstrual periods, bleeding between periods and abdominal or lower back pain.

And in some cases, fibroids can make it hard for a woman to get pregnant, or increase her risk of pregnancy complications like miscarriage and preterm birth.

Since fibroids are so common, "it's very important to understand the societal costs," said researcher Dr. James H. Segars of the U.S. National Institute of Child Health and Human Development in Bethesda, Maryland.

Based on his team's estimates, uterine fibroids cost the U.S. anywhere from $6 billion to $34 billion each year. That's with both treatment costs and "indirect" costs, like lost work days, taken into account.

In fact, Segars said, the cost of lost work -- days off due to fibroid symptoms or to receive treatment -- turned out to be the biggest expense. The researchers put that cost at anywhere from $1.5 billion to $17 billion annually.

Those cost ranges are very wide, Segars acknowledged. But, he said, they are estimates based on a collection of past studies that varied in how they collected data.

"That was the range in the literature," Segars said. He added that, given how common fibroids are, he would guess that the actual costs are closer to the higher end of his team's estimates.

"But it's very difficult to get at the true cost," Segars said.

The findings, reported in the American Journal of Obstetrics & Gynecology, are based on figures from recent studies and national databases.

The researchers estimate that the yearly cost of fibroid treatment in the U.S. ranges from about $4 billion to $9 billion.

Those treatment options include surgery, with hysterectomy (removal of the uterus) being most common. Other women choose to have just the fibroids removed -- a procedure called a myomectomy.

Other procedures include uterine artery embolization, where tiny particles are injected into blood vessels leading to the uterus, to cut off fibroids' blood supply and shrink them; and endometrial ablation, which destroys the lining of the uterus.

Some women use medications -- like painkillers and hormonal therapies -- to manage heavy bleeding and painful periods. But they don't get rid of the fibroids.

As for pregnancy complications linked to fibroids, Segars' team put the yearly cost at anywhere from $238 million to close to $8 billion -- again, a wide range.

Past studies, Segars said, have not considered pregnancy issues in estimating the cost of fibroids. He and his colleagues based their calculations on the prevalence of complications like miscarriage, preterm birth and cesarean section among U.S. women, plus estimates of how many pregnant women have fibroids.

It's been estimated that anywhere from about 0.4 percent to 11 percent of pregnancies in the U.S. each year are to women with uterine fibroids.

Though precise dollar figures are hard to pin down, Segars said the bottom line is that uterine fibroids appear much more costly than has been appreciated.

And it's particularly concerning, he noted, that fibroids are more common and more severe in African-American women -- who may find it harder to bear the burden of lost wages and other costs.

"This most severely affects the population that's least able to bear the costs," Segars said.

That, he added, underscores the importance of finding ways to prevent fibroids.

No one is sure how to do that, however, because researchers don't know exactly why fibroids form. Black women are at greater risk, as are women who are overweight or obese. On the other hand, women who've given birth seem to have a lower risk than those who've never had a baby.