A decade-long study conducted at the Vanderbilt University Medical Center shatters long-held beliefs about uterine fibroids being a cause of miscarriages.

“We find women with fibroids are not at increased risk of miscarriage,” said lead author Katherine Hartmann, MD, PhD. “Women with fibroids had identical risk of miscarriage as women without fibroids when taking into account other risks for pregnancy loss. We were stunned.”

Uterine fibroids are benign smooth muscle tumors found in the uterus.  Their size varies from seedlings undetectable by the human eye up through masses so large that they can distort and enlarge the uterus.  Due to this, they have long been counted among the risk factors for miscarriages.

The researchers put together the largest cohort – more than 5,500 women – ever assembled to investigate the association of fibroids with miscarriage. Ultrasound detected fibroids in 11 percent of the study participants; 89 percent had none. But the chance for miscarriage in both groups turned out to be 11 percent.

“The key message is that fibroids don’t seem to be linked to miscarriage,” Hartmann said.

The original purpose of the study was was to understand which fibroids were responsible for the highest risk of miscarriage in order to determine who might benefit most from surgery or myomectomy to remove the fibroids before a future pregnancy. It was a happy surprise to learn of fibroids' non-role in miscarriages.

The Vanderbilt U study succeeded where previous investigations came up short in part due to the scientists' use of ultrasound; very few of the earlier studies used it. Also, none of the previous cohorts factored into account the influence of age and race – both of which have a known impact.

“More than 1 million miscarriages occur in the U.S. each year,” Hartmann noted. “Loss is remarkably common, but we know very little about the causes. When something bad happens in a pregnancy the first thing women look at is themselves, asking why it happened and what they could have done differently... women with fibroids have one less thing to worry about.”

The study was published in the American Journal of Epidemiology.