metformin during pregnancy

A recent study published in the BMJ investigated whether using metformin during pregnancy leads to birth defects.

Metformin is used for many conditions. The most common use is for type 2 diabetes. With the rise in diagnoses of diabetes in younger adults, an increase in the use of metformin during pregnancy has been reported within the last ten years. Metformin is also used in gestational diabetes, a type of diabetes that develops during pregnancy, and polycystic ovary syndrome, a disease of the ovaries that can affect a woman’s chances of getting pregnant. It is known that metformin crosses the human placenta and the baby is exposed to the medication taken by the mother. This drug can affect cell function in the growing fetus but it is unknown whether this exposure leads to birth defects in babies.

Previous studies have found no evidence that metformin causes birth defects; however, these studies included a small number of participants and were not well designed. Also, studies on whether metformin or other anti-diabetic medications cause birth defects are always complicated because diabetes in the mother is a risk factor for birth defects. If a mother’s blood sugar is not controlled during pregnancy she is putting her baby at risk of birth defects. Researchers at the Administrative Data Research Centre in Northern Ireland conducted a large, international study to investigate whether metformin exposure during pregnancy causes birth defects.

The researchers collected data from over 50,000 babies with birth defects. The risk of birth defects was increased if the mother was diabetic. However, the risk of birth defects was not increased in women with infertility or polycystic ovary syndrome. Only 168 babies in the study were exposed to metformin through the mother’s placenta.

The researchers found no evidence of an increased risk of birth defects in the babies exposed to metformin. The only birth defect that remained significant after the statistical analysis was pulmonary valve atresia, a specific abnormality of the heart. This type of birth defect is associated with diabetes and may not be associated with the medication exposure. Also, the low number of cases of this type of birth defect might also suggest that it occurred by chance.

In this study, the researchers did not find an increase in birth defects in babies born from women who were taking metformin during pregnancy. Abnormalities that were detected were likely due to chance. Further studies are required to confirm the low risk or absence of risk of cardiac abnormalities in babies exposed to metformin during pregnancy.

Written by Jessica Caporuscio, PharmD


  1. Given JE, Loane M, Garne E, et al.Metformin exposure in first trimester of pregnancy and risk of all or specific congenital anomalies: exploratory case-control study. BMJ. 2018.
  2. Friel, LA. Diabetes Mellitus in Pregnancy. Merck Manual.
  3. Pinkerton, JV. Polycystic Ovary Syndrome (PCOS). Merck Manual.
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