Metformin in Pregnancy: An Option for Gestational Diabetes

A study of women with problematic pregnancies finds that an old drug is safe for a newfound use.

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Until now, pregnant women with gestational diabetes had limited treatment options. At one time, excessively high blood-sugar levels during pregnancy were controlled primarily by modifying a patient's diet or, if that failed, using insulin to treat the condition.

Now, there are two other drugs women who have this condition should consider. Glyburide, an oral medication that can be bought for less than $15, has been a viable treatment option for pregnant women since 2000, and metformin, another oral drug that's slightly more expensive, is an option newly deemed safe and effective in a study being published tomorrow in the New England Journal of Medicine. In addition, dietary changes remain important, since they can reduce the risk of having a large baby and a more difficult delivery. The babies of mothers affected by gestational diabetes often overproduce insulin, which may increase an infant's future risk of type 2 diabetes and obesity.

The addition of metformin to the list of options for pregnant women is important, since pills are helpful for women who are wary of giving themselves shots of insulin, says Michael F. Greene, director of obstetrics at Massachusetts General Hospital and associate editor of the New England Journal of Medicine. Greene coauthored an editorial that discusses the metformin research.

Importantly, the new study found that metformin treatment was not associated with increased risks to the baby, compared with insulin. Women in the study also said they preferred metformin over insulin. Both drugs are already used as treatments for type 2 diabetes in adults, but prior to this week's study it wasn't known whether metformin was effective in gestational diabetes—or whether it might cause problems for the fetus, such as too-low blood sugar, respiratory distress, or birth trauma.

Pregnant women who are worried about gestational diabetes may now find that their doctors suggest metformin as an option for treatment. "Up until now people have been reluctant or hesitant to use metformin," says Steven Gabbe, dean of the Vanderbilt School of Medicine, who was not involved in the study. "I think this is now going to change treatment patterns."

While doctors say they're hopeful that having metformin as a treatment option will reduce the number of pregnant women who need to rely on insulin, taking metformin may not be effective enough for them to avoid insulin altogether. About 46 percent of the women in the new study who received metformin also needed supplemental insulin.

It's also not clear if the drug crosses the placenta to reach the baby. The current study shows that whether or not the medicine reaches the baby, there doesn't seem to be an immediate negative impact on the fetus, Gabbe says. Still, he adds, babies whose mothers have taken metformin during pregnancy will need to be followed for a longer period to rule out the risk of long-term problems.

That's enough of a concern to give at least one doctor pause over whether to prescribe metformin for his patients. "Insulin doesn't cross the placenta, and most studies suggest that glyburide doesn't cross the placenta," says Donald Coustan, a professor and chair of obstetrics and gynecology at the Warren Alpert Medical School of Brown University. But the same can't be said for metformin, he says. "I won't be using it because of my concerns about metformin.... That's a judgment call."