Is Metformin Safe in Pregnancy? What the Guidance Says

If you take metformin and you are pregnant or planning to be, you may be asking whether it is safe to continue, or whether you should switch to something else. The reassuring short answer from leading sources is that metformin has been widely used in pregnancy and is not expected to raise the chance of birth defects. At the same time, decisions about diabetes and blood-sugar medicines in pregnancy are individual, and clinicians sometimes prefer insulin. This article gathers what authoritative sources say so you can have a better-informed conversation with the clinician who manages your pregnancy. It is general education only and is not a substitute for that conversation.

What metformin is and what it treats

Metformin is an oral medicine used to lower blood sugar. According to MotherToBaby, it is used to treat type 2 diabetes, insulin resistance in polycystic ovary syndrome (PCOS), and obesity, and it is also used for gestational diabetes, a form of diabetes that appears during pregnancy. It works mainly by lowering the amount of sugar the liver releases and by helping the body respond better to its own insulin. Because it addresses blood sugar and insulin resistance rather than adding insulin directly, metformin is sometimes used both before and during pregnancy in people with these conditions.

What MotherToBaby says about metformin in pregnancy

MotherToBaby, a service of the non-profit Organization of Teratology Information Specialists, is a helpful starting point because it reviews the available research on specific exposures in pregnancy.

Birth defects

MotherToBaby reports that metformin is not expected to increase the chance of birth defects. It also makes an important related point: uncontrolled diabetes in the first trimester can itself increase the chance of birth defects. In other words, keeping blood sugar well managed is part of protecting the pregnancy, which is one reason treatment is emphasized rather than avoided.

Miscarriage

On miscarriage, MotherToBaby states that metformin is not expected to increase the chance of miscarriage. It again notes that poorly controlled diabetes may increase the chance of miscarriage, underscoring that the underlying condition, if left unmanaged, is itself a source of risk.

Effects on the baby and pregnancy

MotherToBaby indicates that studies following children exposed to metformin have not shown clear long-term effects. It is candid, though, that research on some outcomes remains mixed. For questions such as preterm delivery and birth weight, some studies have suggested a difference and others have not. This kind of uncertainty is exactly what your care team weighs when choosing a treatment plan for you.

Metformin versus insulin in pregnancy

A common question is why some clinicians recommend insulin instead of, or in addition to, metformin. Insulin does not cross the placenta in a meaningful way and has a very long track record in pregnancy, so it is frequently treated as the first-choice medicine for managing diabetes during pregnancy when medication is needed. Metformin does reach the baby, and while the available research has been generally reassuring about safety, the long-term data are still being studied. Some people use metformin alone, some use insulin alone, and some use both. The right approach depends on the type of diabetes, how well blood sugar is controlled, and your individual situation, which is why this is a decision to make with your OB or a maternal-fetal-medicine (MFM) specialist rather than on your own.

The other side: untreated high blood sugar carries risk

It is important not to read any caution about medication as a reason to leave diabetes untreated. Poorly controlled blood sugar in pregnancy is associated with serious risks, which can include birth defects when sugar is high early in pregnancy, larger-than-average babies, delivery complications, and problems for the newborn. This is a central reason clinicians work hard to keep blood sugar in a healthy range during pregnancy. The goal of care is safe, effective blood-sugar control, not avoiding all medicine.

Do not start or stop metformin on your own

If you are worried, the safe move is to talk with your prescriber rather than changing anything yourself. Stopping a diabetes medicine without a plan can allow blood sugar to climb, which carries its own risks in pregnancy, and starting a medicine without guidance is not advisable either. Your team can decide whether to continue metformin, adjust it, add insulin, or make another change, and can arrange the blood-sugar monitoring that guides those decisions.

A note on drug “categories”

You may see older references sorting pregnancy medicines into letter categories such as A, B, C, D, and X. Those FDA letter categories have been retired. They were replaced with a narrative labeling system (the Pregnancy and Lactation Labeling Rule) that describes what is actually known from studies rather than assigning a single letter. If you come across an old “category” for metformin, treat it as outdated and rely on current guidance and your care team.

Breastfeeding

MotherToBaby reports that metformin passes into breast milk in very small amounts, and that studies have not found problems in breastfed infants. It notes that extra caution may be warranted for a newborn or an infant with kidney problems. As always, mention any medication to your clinician and your baby’s clinician, and report anything that seems unusual.

When to call your OB or a specialist

Contact your obstetric provider if your blood sugar is running high or is difficult to control, if you have frequent low blood sugars, or if you are unsure whether to continue your medicine. If your diabetes is complex, your provider may involve a maternal-fetal-medicine specialist and a diabetes team. For questions about a specific medication exposure in pregnancy or breastfeeding, you can also call MotherToBaby at 1-866-626-6847, a free service that answers questions about medicines and other exposures.

Frequently asked questions

Is metformin safe to take while pregnant? Metformin has been widely used in pregnancy, and MotherToBaby reports it is not expected to increase the chance of birth defects or miscarriage. Whether it is right for you is a decision for your care team.

Why do some doctors prefer insulin instead? Insulin does not meaningfully cross the placenta and has a very long track record in pregnancy, so it is often the first choice when medication is needed for diabetes. Some people use metformin, some use insulin, and some use both.

Can I keep taking metformin for PCOS during pregnancy? That depends on your situation. Some clinicians continue metformin in pregnancy and some do not. Do not stop or continue it on your own; ask the clinician who manages your pregnancy.

Does metformin reach the baby? Yes, metformin does cross to the baby, unlike insulin. Available research has been generally reassuring about birth defects, though some longer-term data are still being studied.

Can I breastfeed while taking metformin? MotherToBaby reports only very small amounts pass into breast milk and problems have not been found in breastfed infants. Confirm with your clinician, especially for a newborn or a baby with kidney concerns.

What if my blood sugar is still high on metformin? Contact your provider. They may adjust the dose, add insulin, or change the plan, and arrange closer monitoring. Do not change the dose yourself.

Sources

• MotherToBaby (Organization of Teratology Information Specialists) — Metformin fact sheet.
• MedlinePlus (U.S. National Library of Medicine) — general drug and diabetes information.
• American College of Obstetricians and Gynecologists (ACOG) — general guidance on managing diabetes in pregnancy, as summarized in the medical literature.