Is Metformin safe for use during pregnancy?

Mary Loeken Ph.D.

Metformin fully crosses the placenta and achieves levels that are equivalent to Mom’s levels of metformin, so it’s not just a little bit crossing the placenta, it’s full passage of metformin across the placenta, says  Florence Brown, M.D., Co-director of the Joslin-Beth Israel Deaconess Diabetes in Pregnancy Program.

“It is known that there are metformin transporters on placental cells, so metformin could affect the function of the placenta, which is important for delivery of nutrients and hormones to the fetus,” says Dr. Loeken.

This review highlights an issue that still needs to be addressed: What are the long-term effects on the fetus if the mother takes metformin in the second and third trimesters of pregnancy? Fetal cells are more mature than embryo cells, but it is not known whether fetal liver, muscle, or fat tissues express metformin transporters. Further studies are needed to find out.

The question still remains how metformin might affect the fetus and the placenta. “That means we do not know if metformin will affect gene expression, protein synthesis, and if it might affect the long term metabolic profile of the offspring,” says Dr. Brown.

Taken together, the studies included in this review suggest that metformin is not unsafe if taken in the early months of pregnancy, and is not associated with birth defects or miscarriages. One study even found that women on metformin for gestational diabetes have reduced weight gain and fewer episodes of severe low blood sugars, compared to women on insulin. Overall, outcomes for metformin and insulin used for gestational diabetes are similar at delivery.

Even so, Dr. Loeken says there is not enough evidence to recommend using metformin as the sole agent for the management of gestational diabetes or type 2 diabetes during pregnancy.

“Using metformin as much as we do, this review underscores the fact that we need more information about the molecular effect of metformin on the long-term health and metabolism of offspring,” says Dr. Brown.

Here’s what you need to know: 

When diabetes and pregnancy intersect, doctors and patients have to make decisions about the best treatment options to optimize outcomes for both mother and baby. Unhealthy blood glucose levels during pregnancy can lead to birth defects and maternal and fetal complications.

At Joslin, women who are diagnosed with gestational diabetes are only treated with insulin.

“When women with pre-existing diabetes or polycystic ovary syndrome come into our clinic on metformin or glyburide, we will transition them to insulin. We taper them off their oral medications, get them on insulin, and then maintain them on insulin throughout their pregnancy,” says Dr. Brown.

“For women who have been taking metformin before getting pregnant and then become pregnant, there is no evidence that it is unhealthy for your embryo, but you should check with your doctor as soon as possible about continuing on metformin or switching to insulin”, says Dr. Loeken.

If you have diabetes and are pregnant, or are thinking about becoming pregnant, visit the Pregnancy Program’s webpage for more information or to make an appointment.

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