Is it appropriate to continue metformin alongside insulin therapy during the first trimester of pregnancy?

Metformin Should Be Discontinued by the End of the First Trimester When Used Alongside Insulin in Pregnancy

The most recent American Diabetes Association guidelines (2023) explicitly recommend that metformin, when used to treat polycystic ovary syndrome and induce ovulation, should be discontinued by the end of the first trimester. 1 For women with preexisting type 1 or type 2 diabetes requiring insulin therapy, insulin is the preferred agent for management throughout pregnancy. 1

Clinical Reasoning and Evidence Base

For Preexisting Type 2 Diabetes

• Insulin is the preferred medication for treating hyperglycemia in both type 1 and type 2 diabetes during pregnancy. 1
• While one RCT showed that adding metformin to insulin in type 2 diabetes resulted in less maternal weight gain and fewer cesarean births, there was a concerning doubling of small-for-gestational-age neonates. 1
• The 2023 ADA Standards of Care state that insulin should be used to manage type 1 diabetes and is the preferred agent for type 2 diabetes in pregnancy. 1

For Gestational Diabetes

• Metformin and glyburide should not be used as first-line agents for gestational diabetes, as both cross the placenta to the fetus. 1
• Insulin is the preferred medication for treating hyperglycemia in gestational diabetes mellitus. 1
• Lifestyle behavior change is essential and may suffice as treatment for many individuals, with insulin added if needed to achieve glycemic targets. 1

Critical Safety Concerns with Metformin Continuation

Metformin crosses the placenta readily, with umbilical cord blood levels equal to or higher than simultaneous maternal levels. 1

Long-term offspring concerns are particularly troubling:

• Follow-up studies at 4 years showed higher BMI and increased obesity in offspring exposed to metformin. 1
• At 5-10 years, offspring had higher BMI, weight-to-height ratios, waist circumferences, and borderline increased fat mass. 1
• A meta-analysis concluded that metformin exposure resulted in smaller neonates with acceleration of postnatal growth resulting in higher BMI in childhood. 1

Specific Contraindications for Metformin in Pregnancy

Metformin should absolutely not be used in pregnant women with:

• Hypertension or preeclampsia 1
• Risk for intrauterine growth restriction 1
• Placental insufficiency (due to potential for growth restriction or acidosis) 1

When Metformin Was Used for PCOS/Ovulation Induction

• Randomized controlled trials comparing metformin with other therapies for ovulation induction in women with polycystic ovary syndrome have not demonstrated benefit in preventing spontaneous abortion or gestational diabetes. 1
• There is no evidence-based need to continue metformin in such patients once pregnancy has been confirmed. 1
• The 2023 guidelines explicitly state: "Metformin, when used to treat polycystic ovary syndrome and induce ovulation, should be discontinued by the end of the first trimester." 1

Clinical Algorithm for First Trimester Management

If metformin was started for PCOS/ovulation induction:

• Discontinue by end of first trimester 1
• Transition to insulin if glycemic control requires pharmacotherapy 1

If patient has preexisting type 2 diabetes on metformin:

• Transition to insulin as the preferred agent 1
• Screen for contraindications (hypertension, preeclampsia risk, growth restriction risk) 1
• If metformin continuation is considered due to barriers to insulin use (cost, language, comprehension, cultural influences), this requires thorough discussion of known risks and lack of long-term safety data 1

If patient develops gestational diabetes:

• Start with lifestyle modification 1
• Add insulin if needed, not metformin as first-line 1

Common Pitfalls to Avoid

• Do not continue metformin throughout pregnancy based solely on short-term glycemic benefits, as long-term offspring metabolic effects are concerning 1
• Do not use metformin in any pregnant woman with hypertension, preeclampsia, or fetal growth concerns 1
• Do not assume metformin is equivalent to insulin for pregnancy management—the evidence consistently shows insulin as preferred 1
• Do not rely on older studies or practices—the most recent 2023 ADA guidelines provide the clearest directive on discontinuation 1

FDA Labeling Perspective

The FDA label for metformin states that limited data with metformin in pregnant women are not sufficient to determine a drug-associated risk for major birth defects or miscarriage, and that published studies have not reported a clear association with major birth defects or miscarriage risk. 2 However, the label emphasizes that there is insufficient information and that developmental and health benefits of alternative treatments should be considered. 2