Is Metformin Safe During Pregnancy? A Clinical Evidence Review

TL;DR

• Metformin crosses the placenta and reaches fetal plasma concentrations comparable to maternal levels, making its use in pregnancy a nuanced clinical decision [2].
• For the mother, metformin can reduce insulin requirements, weight gain, hypoglycemia risk, and possibly pre-eclampsia; however, it is not universally recommended as first-line therapy during pregnancy [2][4].
• Short-term neonatal outcomes appear reassuring (no increased congenital anomalies), but long-term offspring safety data remain limited, with some signals of concern regarding altered growth trajectories [1][2].
• Major guidelines differ: the European Union approved metformin for diabetes in pregnancy in 2022, while U.S. practice generally favors insulin as first-line pharmacotherapy [2][3].
• Women taking metformin who become pregnant should consult their prescriber immediately rather than stopping the medication abruptly.

Why Metformin Use in Pregnancy Is a Growing Question

Metformin (Glucophage) is a synthetic biguanide and one of the most widely prescribed medications worldwide. It is the established first-line oral therapy for type 2 diabetes mellitus (T2DM) owing to its robust glucose-lowering effect, well-characterized safety profile, and low cost [7]. Beyond T2DM, metformin is extensively used off-label in polycystic ovary syndrome (PCOS) to address insulin resistance, hyperandrogenism, and anovulatory infertility [5][6]. Because many women of reproductive age take metformin—either for diabetes or PCOS—the question of whether to continue, start, or stop it once pregnancy is confirmed arises frequently in clinical practice.

The topic is not straightforward. Metformin freely crosses the placenta, achieving fetal plasma concentrations comparable to those in the mother at delivery [2]. This pharmacokinetic reality sets it apart from insulin, which does not cross the placenta in clinically meaningful amounts. As a result, any discussion of metformin safety in pregnancy must consider two patients: the mother and the developing fetus, as well as the child's long-term health trajectory after birth.

This article synthesizes the best available evidence—including randomized controlled trials (RCTs), systematic reviews, and guideline recommendations—to help patients and clinicians navigate this decision.

How Metformin Works: Mechanism of Action Relevant to Pregnancy

At clinically relevant plasma concentrations (50–100 μM), metformin primarily lowers blood glucose by inhibiting hepatic gluconeogenesis through a redox-dependent mechanism [7]. It alters the cellular NAD⁺/NADH ratio, selectively suppressing the conversion of lactate and glycerol into glucose in the liver [7]. While earlier research emphasized AMP-activated protein kinase (AMPK) activation as the dominant pathway, more recent evidence indicates that robust AMPK activation occurs mainly at supra-pharmacological concentrations (>1 mM) unlikely to be achieved in routine clinical use [7].

In the context of pregnancy, metformin's insulin-sensitizing properties are particularly relevant. Pregnancy itself is a state of progressive insulin resistance—driven by placental hormones such as human placental lactogen, progesterone, and cortisol—that ensures adequate glucose delivery to the growing fetus. When this physiological insulin resistance becomes pathological, gestational diabetes mellitus (GDM) develops. GDM is the most common medical complication of pregnancy, and maintaining adequate blood glucose control reduces morbidity for both mother and baby [3].

Metformin addresses insulin resistance at its root rather than simply supplementing insulin supply, which explains its appeal as a therapeutic option. In women with PCOS, metformin improves insulin sensitivity, menstrual regularity, and androgen levels, and may enhance fertility when combined with other treatments such as clomiphene citrate or letrozole [5].

Metformin in Pregnancy: Indications and Clinical Scenarios

There are several distinct clinical contexts in which metformin may be used during pregnancy, and the risk–benefit balance differs for each.

Gestational Diabetes Mellitus (GDM)

GDM affects an estimated 5–20% of pregnancies globally, depending on the diagnostic criteria and population studied [3]. The initial management of GDM consists of medical nutrition therapy (diet modification) and physical activity. When these lifestyle measures fail to achieve glycemic targets—typically within one to two weeks—pharmacotherapy is initiated [3][4].

Insulin has traditionally been considered the preferred pharmacological agent for GDM because it does not cross the placenta [3][4]. However, insulin therapy requires injection, refrigeration, dose titration, and carries a risk of maternal hypoglycemia. Metformin offers a practical oral alternative. Multiple RCTs, including the landmark MiG (Metformin in Gestational diabetes) trial, have demonstrated that metformin is effective at achieving glycemic control in GDM, though approximately 30–46% of women initially assigned to metformin require supplemental insulin to reach targets [4].

While short-term efficacy data are encouraging, metformin is not considered a universal first-line agent for GDM because it crosses the placenta to the fetus, and long-term offspring safety data remain insufficient [4]. Both the American College of Obstetricians and Gynecologists (ACOG) and the American Diabetes Association (ADA) acknowledge metformin as a reasonable alternative to insulin in GDM when the patient cannot use or refuses insulin, but insulin remains the preferred recommendation [VERIFY].

Notably, the European Union took a different position in 2022, approving metformin as the only oral antidiabetic medication for diabetes in pregnancy [2].

Pre-existing Type 2 Diabetes in Pregnancy

Women with T2DM who become pregnant while taking metformin face an immediate clinical question: should the drug be continued? Evidence from observational studies and some RCTs suggests that metformin does not increase the risk of major congenital anomalies when used in the first trimester [2]. However, most guidelines recommend transitioning to insulin for glycemic management during pregnancy in women with T2DM, given the more predictable dose–response relationship and the absence of placental transfer [VERIFY].

Some clinicians opt to continue metformin as an adjunct to insulin during pregnancy in women with T2DM, particularly when insulin requirements are very high or when excessive weight gain is a concern [2].

Polycystic Ovary Syndrome (PCOS)

PCOS is the most common endocrine disorder in women of reproductive age and accounts for more than 75% of cases of anovulatory infertility [6]. Many women with PCOS conceive while taking metformin for fertility enhancement or metabolic management. The question then becomes whether to continue metformin through the first trimester or the entire pregnancy.

Several observational studies and small RCTs have reported that metformin use in early pregnancy in women with PCOS may reduce the rate of early pregnancy loss and miscarriage [5][6]. A comprehensive review by Palomba et al. (2025) found that clinical evidence supports metformin's efficacy in reducing the risk of preterm birth and pregnancy-induced hypertension in PCOS pregnancies, but the benefits for miscarriage or GDM prevention remain unclear [1]. Similarly, Saadati et al. (2025) noted that while metformin may reduce early pregnancy loss, miscarriage, and preterm birth in PCOS, findings for gestational diabetes and pre-eclampsia prevention are inconsistent [5].

The use of metformin as a strategy for preventing pregnancy complications in women with PCOS remains inconsistent due to limited consensus and concerns about long-term offspring safety [1].

Obesity in Pregnancy

Some researchers have investigated metformin use in pregnant women with obesity (without diabetes) to reduce excessive gestational weight gain and associated complications. While some trials have shown modest reductions in weight gain, the evidence for clinically meaningful improvements in maternal or neonatal outcomes is not yet sufficient to support routine use in this population [2].

Metformin Dosing and Comparison with Other Agents in Pregnancy

Glyburide (known as glibenclamide outside the United States) should be used with caution during pregnancy. It crosses the placenta and has been associated with increased risk of neonatal hypoglycemia and, in some studies, macrosomia [4]. Current expert opinion generally favors metformin over glyburide when an oral agent is chosen for GDM [4].

Maternal Benefits of Metformin During Pregnancy

For the mother, the evidence base for metformin is broadly favorable:

• Reduced insulin therapeutic burden. Metformin can reduce the total daily insulin dose required in women who need combination therapy, and may eliminate the need for insulin altogether in some GDM patients [2].
• Less weight gain. Metformin-treated women consistently gain less weight during pregnancy compared with those on insulin alone, which may have downstream benefits for delivery outcomes and postpartum weight retention [2].
• Lower risk of maternal hypoglycemia. Because metformin does not directly stimulate insulin secretion, the risk of hypoglycemic episodes is substantially lower than with insulin or sulfonylureas [2][7].
• Possible reduction in pre-eclampsia. Some trials have suggested that metformin may reduce the risk of pre-eclampsia, though findings are inconsistent across studies and populations [2][5].
• Reduction of preterm birth risk in PCOS. In women with PCOS, metformin administration during pregnancy has been associated with reduced rates of preterm delivery and pregnancy-induced hypertension [1][5].

These maternal benefits must be weighed against the central concern: metformin reaches the fetus.

Adverse Effects and Safety Considerations of Metformin in Pregnancy

The Offspring Question: What We Know and What We Don't

The most consequential uncertainty surrounding metformin use in pregnancy concerns long-term offspring outcomes. Multiple follow-up studies from major RCTs have attempted to address this question, with results that are cautiously reassuring in the short term but raise some flags at longer follow-up.

Malek and Davis (2025) conducted a comprehensive review of offspring follow-up data from RCTs of metformin use in GDM, T2DM, PCOS, and obesity-related pregnancies [2]. Key findings include:

• Neonatal outcomes: No increased risk of congenital anomalies. Some reduction in neonatal hypoglycemia compared with insulin. Birth weight effects vary by indication—in GDM, metformin-exposed neonates tend to be lighter; in PCOS, effects on birth weight are inconsistent [2].
• Childhood growth: Some follow-up studies (notably the MiG TOFU study at ages 2 and 7–9 years) have reported that children exposed to metformin in utero may have higher BMI or increased subcutaneous fat, though total body fat percentage may not differ [2]. These findings have generated concern about potential metabolic programming effects.
• SGA risk: There is a signal suggesting that metformin exposure may be associated with smaller birth size in some contexts, which could have implications for long-term cardiometabolic health [2].

Palomba et al. (2025) echoed these concerns, stating that the use of metformin during PCOS pregnancies remains limited by concerns about long-term offspring safety [1]. The authors noted that methodological differences and heterogeneous patient profiles across studies make it difficult to draw firm conclusions [1].

It is essential to acknowledge that the absence of clear harm is not the same as proof of safety. Follow-up periods in most studies extend only to early childhood, and the metabolic consequences of in-utero metformin exposure may not become apparent until adolescence or adulthood.

Special Populations and Clinical Pearls

First Trimester Exposure

Many women with PCOS or T2DM are taking metformin at the time of conception and may continue it through the first trimester before pregnancy is confirmed. Available evidence does not suggest an increased risk of major birth defects with first-trimester metformin exposure [2][6]. This is reassuring for women who discover they are pregnant while taking the medication, but it does not constitute a blanket endorsement for intentional first-trimester use.

Women with PCOS Undergoing Fertility Treatment

In women with PCOS using metformin as part of a fertility treatment regimen, the decision about when to discontinue metformin after conception varies by clinician and clinical context. Some reproductive endocrinologists continue metformin through the first trimester to reduce miscarriage risk, then taper and discontinue [5][6]. Others discontinue at the time of a positive pregnancy test. Neither approach has been validated by definitive RCT evidence, and practice patterns reflect individual clinical judgment and patient preferences.

Breastfeeding

Metformin is excreted in breast milk in small amounts (generally less than 1% of the weight-adjusted maternal dose). The American Academy of Pediatrics (AAP) considers metformin compatible with breastfeeding [VERIFY]. Infant exposure via breast milk is far lower than the placental exposure during pregnancy.

Renal Function Monitoring

Pregnancy itself causes hemodynamic changes that affect renal function, including increased glomerular filtration rate (GFR). While this typically enhances metformin clearance, clinicians should monitor renal function periodically and discontinue metformin if eGFR drops below 30 mL/min/1.73 m² or if acute kidney injury develops (e.g., in the setting of severe pre-eclampsia or hemorrhage).

Perioperative Considerations

If a cesarean delivery is planned, metformin should typically be withheld on the day of surgery and restarted once oral intake is established and renal function is confirmed to be stable. This aligns with general perioperative metformin guidance.

Key Guideline Positions

• ACOG (USA): Insulin is preferred for GDM and T2DM in pregnancy. Metformin is an acceptable alternative when insulin is not feasible or the patient declines [VERIFY].
• NICE (UK): Metformin may be offered as first-line pharmacotherapy for GDM if blood glucose targets are not met with lifestyle modifications. Insulin should be added if targets remain unmet [VERIFY].
• EMA (EU): Metformin was approved for use in diabetes during pregnancy in 2022 [2].
• WHO: Recommends insulin as the treatment of choice for GDM. Metformin may be considered where insulin is unavailable or unacceptable to the patient [VERIFY].

FAQ

Q1: Can I continue taking metformin if I find out I am pregnant? A1: Do not stop metformin abruptly without consulting your healthcare provider. Current evidence does not show an increased risk of birth defects from first-trimester metformin exposure [2][6]. However, your clinician will assess your specific situation—including the reason you are taking metformin, your blood glucose levels, and your pregnancy risk factors—to decide whether to continue metformin, switch to insulin, or use a combination approach. The decision should be individualized.

Q2: Is metformin safer than insulin during pregnancy? A2: "Safer" depends on what outcome is being measured. For the mother, metformin carries a lower risk of hypoglycemia and excessive weight gain compared with insulin [2]. It is also easier to administer (oral vs. injection). However, insulin does not cross the placenta, while metformin does, reaching the fetus at concentrations comparable to the mother's [2][4]. The long-term effects of this fetal exposure are not fully understood. For this reason, most guidelines still consider insulin the preferred agent, while acknowledging metformin as a reasonable alternative.

Q3: Will metformin harm my baby? A3: Short-term data are reassuring: there is no demonstrated increase in congenital anomalies, and neonatal hypoglycemia may actually be reduced with metformin compared to insulin [2]. However, some follow-up studies have raised questions about subtle effects on childhood growth patterns, such as slightly higher BMI in early childhood [2]. These findings are inconsistent across studies and may not be clinically significant. Long-term follow-up into adolescence and adulthood is still needed [1][2].

Q4: Should I take metformin throughout my entire pregnancy if I have PCOS? A4: There is no universal recommendation. Some evidence suggests that metformin use throughout pregnancy in women with PCOS may reduce the risk of preterm birth and pregnancy-induced hypertension [1][5]. However, the data on miscarriage prevention and GDM prevention are conflicting [1][5]. Discuss the risks and potential benefits with your obstetrician or reproductive endocrinologist, who can tailor advice to your clinical profile.

Q5: Can metformin prevent gestational diabetes? A5: Several studies have investigated whether metformin taken early in pregnancy can prevent GDM in high-risk women (such as those with obesity or PCOS). Results have been mixed, and no intervention appears universally superior to placebo for GDM prevention [4]. Metformin is not currently recommended as a preventive strategy for GDM outside of clinical trials.

References

[1] Palomba S, Seminara G, Costanzi F. Expert Review of Endocrinology & Metabolism 2025. PMID:41086037. pubmed.ncbi.nlm.nih.gov/41086037

[2] Malek R, Davis SN. Expert Opinion on Drug Safety 2025. PMID:39485003. pubmed.ncbi.nlm.nih.gov/39485003

[3] Alfadhli EM. Saudi Medical Journal 2015. PMID:25828275. pubmed.ncbi.nlm.nih.gov/25828275

[4] Chatzakis C, Cavoretto P, Sotiriadis A. Current Pharmaceutical Design 2021. PMID:33550962. pubmed.ncbi.nlm.nih.gov/33550962

[5] Saadati S, Mason T, Godini R. Diabetes, Obesity & Metabolism 2025. PMID:40329601. pubmed.ncbi.nlm.nih.gov/40329601

[6] Brock B, Smidt K, Ovesen P. Basic & Clinical Pharmacology & Toxicology 2005. PMID:15910403. pubmed.ncbi.nlm.nih.gov/15910403

[7] LaMoia TE, Shulman GI. Endocrine Reviews 2021. PMID:32897388. pubmed.ncbi.nlm.nih.gov/32897388

About the author

Dr. Stanislav Ozarchuk, PharmD, has 15 years of clinical pharmacy experience. He writes for PillsCard.com, the international drug encyclopedia.

Medical disclaimer

The information provided here is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any medication.