Trondheim, Norway—Metformin, when used during pregnancy, appears to cross the placenta and affect the fetus, increasing a child’s risk of becoming obese or overweight.

That’s according to a surprising study in The Journal of Clinical Endocrinology & Metabolism, which points out that an increasing number of pregnant women take metformin to treat gestational diabetes or polycystic ovary syndrome (PCOS). PCOS alone affects as many as 10% of women of childbearing age, according to background information in the article.

“Our findings indicate the offspring of women who took metformin for PCOS during pregnancy are more likely to meet the criteria for obesity or overweight than children whose mothers were given a placebo during pregnancy,” noted first author Liv Guro Engen Hanem, MD, of the Norwegian University of Science and Technology in Trondheim, Norway. “The results were surprising, since limited past research in this area had suggested metformin would have a protective effect on the children’s metabolic health.”

The study involved 292 children whose parents participated in two previous randomized clinical trials. Pregnant women with PCOS took either metformin or a placebo during pregnancy, in the previous studies.

Researchers discovered that, by age 4, children whose mothers were randomized to metformin during pregnancy tended to weigh more than the children whose mothers received placebo. The effect of metformin on birth weight did not become apparent until the offspring reached 6 months old, and, by age 4 years, the children in the metformin group had higher BMI scores and were more likely to meet the criteria for obesity or overweight than children in the placebo group.

“Few studies have examined the long-term health of children born to women with PCOS who took metformin,” Hanem said. “Our findings indicate more research is needed to determine its effects on children who were exposed in the womb.”

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