Metformin During Pregnancy: Safety, Risks, and Guidelines

Is Metformin Safe During Pregnancy?

Metformin is increasingly used during pregnancy, primarily for gestational diabetes mellitus (GDM) and polycystic ovary syndrome (PCOS). Unlike many medications, [metformin](/drugs/metformin) crosses the placenta and reaches the fetus in concentrations similar to maternal blood levels. Despite this, available evidence suggests metformin does not cause birth defects and is not associated with major congenital malformations.

However, the long-term effects of fetal metformin exposure are still being studied, and some concerns have emerged from follow-up studies. The decision to use metformin during pregnancy should be made with your healthcare provider after weighing the benefits and risks for your specific situation.

FDA Pregnancy Category

Metformin is classified as FDA Pregnancy Category B — meaning animal reproduction studies have not demonstrated a risk to the fetus, and there are no adequate and well-controlled studies in pregnant women. This is a relatively reassuring classification, though it does not guarantee safety.

Metformin for Gestational Diabetes

Gestational diabetes (GDM) affects 6–9% of pregnancies and increases the risk of macrosomia (large baby), birth complications, neonatal hypoglycemia, and long-term risk of [type 2 diabetes](/conditions/type-2-diabetes) in both mother and child. The standard treatment is dietary modification, with insulin added when glucose targets are not met.

Metformin is increasingly used as an alternative or adjunct to insulin for GDM. Key findings from clinical trials:

• MiG trial (2008) — Metformin was as effective as insulin for GDM, with similar rates of neonatal complications. Metformin was preferred by patients due to ease of use and lower cost. However, 46% of metformin-treated patients required supplemental insulin.
• Neonatal outcomes — No increase in congenital malformations, neonatal hypoglycemia, or perinatal mortality with metformin vs. insulin.
• Long-term follow-up (MiG TOFU) — Children exposed to metformin in utero had slightly higher BMI and body fat at age 2 and 7–9 years compared to insulin-exposed children. The clinical significance is uncertain.

Metformin for PCOS in Pregnancy

Women with PCOS have higher rates of miscarriage, gestational diabetes, and preeclampsia. Metformin is sometimes continued throughout pregnancy in women with PCOS to reduce these risks. Evidence is mixed:

• Some studies show reduced miscarriage rates with metformin in PCOS
• Evidence for prevention of GDM in PCOS is inconsistent
• Most guidelines do not recommend routine metformin use throughout pregnancy in PCOS

Current Guidelines

• ADA (American Diabetes Association) — Metformin is an acceptable alternative to insulin for GDM when insulin is not available or acceptable to the patient, with the caveat that metformin crosses the placenta and long-term safety data are limited.
• ACOG (American College of Obstetricians and Gynecologists) — Insulin is the preferred agent for GDM; metformin and glyburide are alternatives but should not be considered equivalent to insulin.
• NICE (UK) — Metformin is a first-line option for GDM alongside dietary modification.

References

1. Rowan JA, et al. Metformin versus insulin for the treatment of gestational diabetes (MiG). N Engl J Med. 2008;358(19):2003-2015.
2. Hanem LGE, et al. Metformin use in PCOS pregnancies increases the risk of offspring overweight at 4 years of age. J Clin Endocrinol Metab. 2018;103(4):1612-1621.
3. American Diabetes Association. Management of diabetes in pregnancy: Standards of Medical Care in Diabetes — 2024. Diabetes Care. 2024;47(Suppl 1):S282-S294.