Fibroids Don't Cause Miscarriage, Study Finds

A common uterine condition used to be blamed for pregnancy loss, but a new study has disproved the link.

fibroids and miscarriage Valeria Aksakova/Shutterstock
If you've had non-cancerous uterine growths called fibroids and are pregnant or trying to conceive, you're probably worried about how they'll affect your chance of miscarriage. Good news: A major study has found they don't cause pregnancy loss. What a relief!

Reassuring results

The study, published in the American Journal of Epidemiology, looked at 5,500 women from different urban and suburban communities over 10 years to see how fibroids would affect pregnancy. They found that the miscarriage rate in both the group of women with fibroids and the group without was the same, 11 percent. These results are even more surprising because the initial goal for the study was to see what kind of fibroids had the highest risk of miscarriage, so the researchers weren't expecting to find that fibroids didn't increase the risk to begin with.

These findings are heartening for the many women with fibroids—according to the National Institutes of Health, 30 percent of women ages 25 to 44 have symptoms of fibroids (and that's not counting those who don't have symptoms!). But why had previous research shown that fibroids were connected to pregnancy loss? "When a condition like fibroids is common and an event is common like miscarriage, there is risk of the appearance that they are 'true, true, yet unrelated,' meaning we correlate rather than prove the association," lead study author and Vanderbilt University Medical Center professor of obstetrics and gynecology Katherine Hartmann, MD, PhD, tells Fit Pregnancy.

Fibroids tend to increase with age, as do miscarriages. Plus, African-American women are more likely to have fibroids and are more likely to have miscarriages. "Both the effects of age and race/ethnicity confound the relationship between fibroids and loss when not taken into account," Dr. Hartmann says. "Prior studies did not do this, and a large study was required to uncover this." Not taking into account age and race "tangles up the underlying cause of the loss with an 'innocent' bystander, namely fibroids," she says.

Do fibroids affect pregnancy at all?

Previously, common thinking was that because fibroids distort the shape of the uterus, they could make growth of the pregnancy more difficult, Dr. Hartmann says. Fibroids were also thought to increase cramping and inflammatory reactions that could lead to loss, and divert blood supply away from the uterus. "Much of this is inferred and has not been demonstrated in humans," she says.

That's not to say that fibroids have no effect on pregnancies, though. "They do increase risk of bleeding, pain from changing size, malposition of the placenta [such as placenta previa], and they also increase risk of cesarean," Dr. Hartmann says. "Work remains to be done on their influence on preterm birth, premature rupture of membranes and fetal growth."

But even so, the study's results might save a lot of women from undergoing major surgery called a myomectomy to remove fibroids before attempting pregnancy. "That's what we are worried about now—unnecessary intervention," Dr. Hartmann says. "And as we can see in our data, since most women do well in the next pregnancy without intervention, the surgery is perceived by patient and surgeon as having worked." Women with unexplained recurrent losses might want to consider removing fibroids, she says—but even that isn't proven to have any effect. "In that event I would still discuss surgery based on the fibroid location, though we lack data to say this is the right thing to do," Dr. Hartmann says. "Randomized clinical trials in high-risk women show it does not help."

Research will continue to explore the role of fibroids in pregnancy, but for now pregnant women dealing with the condition can put their minds a bit more at ease.