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Metformin and Pregnancy: Safety, Placental Transfer and When to Stop
Can Metformin Be Used During Pregnancy?
Yes, metformin may be used during pregnancy in selected situations. It lowers glucose mainly by reducing glucose production in the liver and improving the body’s response to insulin. Unlike insulin, however, metformin reaches the developing baby through the placenta.
The best treatment depends on why you take metformin, how high your glucose is, the stage of pregnancy, your kidney function, your ability to use insulin safely, and the guidance followed in your country. If you have type 1 diabetes, metformin cannot replace insulin.
The most important point is not to leave high blood sugar untreated. Poorly controlled diabetes during pregnancy increases the risk of miscarriage, preeclampsia, premature birth, a very large baby, difficult delivery, newborn low blood sugar, and birth defects. Your treatment plan should protect both you and your baby from these risks.
Does Metformin Cross the Placenta?
Studies of first-trimester exposure have not found an increased rate of major congenital malformations compared with the underlying risk in women with diabetes or PCOS. Research following children later in life has produced mixed results about weight and body composition, so this remains an area of ongoing study.
Why Do Pregnancy Guidelines Differ?
You may receive different advice depending on where you live and your individual circumstances. That does not necessarily mean one clinician is wrong. Professional groups weigh the known benefits, placental exposure, access to insulin, and uncertain long-term evidence differently.
| Guidance | General position | What it means for you |
|---|---|---|
| American Diabetes Association (ADA) | Insulin is preferred for type 2 diabetes in pregnancy and when medication is needed for gestational diabetes. Metformin is not a first-line medicine because it crosses the placenta and may not achieve glucose targets by itself. | Metformin may still be considered when insulin cannot be used safely or effectively, after informed discussion. |
| UK NICE guidance | Metformin may be used before and during pregnancy as an adjunct or alternative to insulin when better glucose control is expected to outweigh potential harm. | Your clinician may recommend metformin alone, with insulin, or a transition to insulin, depending on your readings. |
When Should I Stop Metformin During Pregnancy?
There is no single stop date that is right for everyone. The reason you take it matters.
| Your situation | Typical clinical discussion |
|---|---|
| Metformin used only for PCOS or to induce ovulation | ADA guidance recommends stopping it by the end of the first trimester. Trials have not shown that continuing it reliably prevents miscarriage or gestational diabetes. |
| Type 2 diabetes before pregnancy | Do not stop suddenly. Your team may continue it temporarily, add insulin, or transition you to insulin. A planned change prevents a dangerous rise in glucose. |
| Gestational diabetes | If lifestyle changes are not enough, insulin is preferred in U.S. guidance. Metformin may be considered after counseling, particularly when insulin is not feasible. Many women still need added insulin. |
| Vomiting, dehydration, severe infection, low oxygen, or worsening kidney function | A clinician may temporarily withhold metformin because these problems can increase the risk of lactic acidosis. Seek instructions promptly. |
| After delivery | Medication needs change quickly. Metformin may be stopped after gestational diabetes or continued for type 2 diabetes or PCOS. Confirm the plan before leaving the hospital. |
Metformin for Type 2 Diabetes in Pregnancy
Pregnancy increases insulin resistance, especially during the second and third trimesters. A dose or treatment that controlled your glucose before conception may no longer be enough. Insulin is often preferred because it does not cross the placenta and can be adjusted as pregnancy changes.
Some clinicians continue metformin with insulin to reduce insulin requirements or maternal weight gain. However, the balance of benefits and uncertainties should be reviewed individually. Metformin is generally avoided when placental insufficiency or fetal growth restriction is a concern, including certain cases involving hypertension or preeclampsia.
For more background, read about how metformin affects insulin resistance and the benefits and side effects of diabetes medicines.
Metformin for Gestational Diabetes
A food plan, safe physical activity, and glucose monitoring are the first steps for many women with gestational diabetes. If these do not keep readings within target, medication is needed. Insulin is the preferred first-line medicine in current U.S. guidance.
Metformin can lower maternal glucose and is easier to take than injections, but it has two important limitations: it crosses the placenta, and it may not be strong enough on its own. Across studies, roughly one-quarter or more of women assigned to metformin required added insulin to reach pregnancy glucose targets.
Metformin also has not reliably prevented gestational diabetes when given to high-risk women with obesity, PCOS, or insulin resistance. If you have had gestational diabetes before, early screening and ongoing monitoring are more useful than assuming metformin will prevent it.
See my practical guides to gestational diabetes treatment, possible signs of gestational diabetes, and the gestational diabetes test.
Should Women With PCOS Continue Metformin?
Metformin is commonly prescribed before pregnancy for insulin resistance, irregular ovulation, or fertility treatment associated with polycystic ovary syndrome (PCOS). Once pregnancy is established, the decision changes.
Current ADA guidance recommends discontinuing metformin used for PCOS and ovulation by the end of the first trimester. Randomized trials have not shown that continued treatment prevents gestational diabetes or early miscarriage. A different plan may be needed if you also have type 2 diabetes, so ask the prescriber to clarify the reason for treatment before stopping.
Safety, Side Effects, and Warning Signs
The most common side effects are nausea, diarrhea, stomach discomfort, gas, and reduced appetite. These may be harder to distinguish from ordinary pregnancy symptoms. Taking metformin with food may help, but follow the instructions given with your prescription.
Metformin alone rarely causes hypoglycemia. Low glucose becomes more likely when it is combined with insulin or another glucose-lowering medicine, meals are missed, or vomiting prevents you from eating.
Lactic acidosis is very rare but serious. The risk rises with severe kidney disease, dehydration, severe infection, low oxygen, or major illness. Contact your pregnancy care team urgently for persistent vomiting, inability to keep fluids down, rapid or difficult breathing, unusual drowsiness, severe weakness, or worsening abdominal pain.
For a broader medicine-safety overview, see metformin (Glucophage) side effects.
Common Glucose Targets During Pregnancy
The ADA lists the following general goals for diabetes in pregnancy. Your own targets may differ, so use the numbers set by your obstetric and diabetes team.
| Timing | Common target |
|---|---|
| Before eating / fasting | Less than 95 mg/dL (5.3 mmol/L) |
| 1 hour after starting a meal | Less than 140 mg/dL (7.8 mmol/L) |
| 2 hours after starting a meal | Less than 120 mg/dL (6.7 mmol/L) |
A1C is useful, but daily glucose readings guide pregnancy treatment more directly. The ADA suggests an A1C below 6% when this can be reached without significant hypoglycemia, with a less strict goal such as below 7% when necessary for safety.
Your Pregnancy Monitoring Checklist
- Record fasting and after-meal glucose at the times your team recommends.
- Review your medication plan early in pregnancy and whenever readings rise.
- Have kidney function checked before and during treatment when advised.
- Attend blood-pressure, preeclampsia, and fetal-growth assessments.
- Ask for a sick-day plan covering vomiting, dehydration, ketones, and when to hold medication.
- Carry a fast-acting glucose source if you also use insulin.
- Discuss vitamin B12 testing if you have used metformin for a long time or have anemia or nerve symptoms.
A balanced eating pattern remains important whether you use metformin or insulin. My guide to carbohydrates, glycemic index, and diabetes explains how meal composition can affect after-meal readings.
Planning Pregnancy While Taking Metformin
If possible, arrange a preconception visit before trying to conceive. Review every medicine and supplement, check kidney and eye health, begin an appropriate folic-acid plan, and work toward your individualized glucose target. ADA guidance recommends an A1C below 6.5% before conception when this can be achieved safely.
If you become pregnant unexpectedly, contact your clinician promptly—but continue essential diabetes treatment until you receive a safe replacement plan.
After Delivery and Breastfeeding
Insulin resistance often falls quickly after delivery, so medication doses may need immediate adjustment. Women who had gestational diabetes usually stop glucose-lowering medication after birth and should have a 75-gram oral glucose tolerance test 4–12 weeks later. Continued lifelong screening is important because the future risk of type 2 diabetes remains higher.
Metformin passes into breast milk in small amounts. Available evidence and NHS guidance consider it compatible with breastfeeding a healthy baby, but confirm the plan if your baby was premature, unwell, or has kidney problems.
What About Metformin Taken by the Father?
A father’s use of metformin is not expected to expose the fetus directly. Earlier observational findings raised questions, but later research has been reassuring overall. People planning a pregnancy should still review medicines with their clinician, especially if diabetes control or fertility is a concern.
Doctor’s Note
When a patient asks me, “Is metformin safe in pregnancy?” I explain that the answer is more nuanced than yes or no. We have reassuring data about major birth defects, but metformin reaches the baby and long-term evidence is still developing. The safest plan is the one that keeps glucose controlled while considering the reason for treatment, kidney health, pregnancy complications, access to insulin, and the patient’s informed preferences. Please do not let fear of medication lead to untreated high blood sugar.
Frequently Asked Questions
1. Is metformin safe during the first trimester?
Available studies have not shown an increased risk of major birth defects from first-trimester exposure. However, the reason for treatment matters. Metformin used only for PCOS or ovulation is generally stopped by the end of the first trimester under ADA guidance, while diabetes treatment must be continued or safely replaced.
2. Can I stop metformin as soon as I discover I am pregnant?
Do not stop it on your own. Contact your clinician promptly. If you take it for diabetes, stopping suddenly can raise glucose during a sensitive stage of fetal development. Your team may continue it temporarily, add insulin, or arrange a transition.
3. Does metformin cross the placenta?
Yes. Metformin crosses the placenta, and fetal blood levels later in pregnancy can be similar to maternal levels. This is one reason insulin is preferred in U.S. guidance when medication is needed.
4. Does metformin prevent gestational diabetes in women with PCOS?
Not reliably. Trials and meta-analyses have not shown that metformin consistently prevents gestational diabetes in women at high risk because of PCOS, obesity, or insulin resistance.
5. Is insulin safer than metformin in pregnancy?
Insulin does not cross the placenta and is preferred by the ADA for type 2 and gestational diabetes in pregnancy. Metformin may still be reasonable in selected cases. Safety also depends on achieving good glucose control and avoiding severe low glucose.
6. Can metformin cause low blood sugar while pregnant?
Metformin alone rarely causes hypoglycemia. The risk is higher when it is combined with insulin or other glucose-lowering medicines, when meals are missed, or during vomiting and poor intake.
7. Will metformin harm my baby or cause birth defects?
Current evidence does not show an increased risk of major congenital malformations. Long-term studies of child growth and metabolism are mixed, so follow-up research is continuing. Uncontrolled diabetes itself has clear risks for both mother and baby.
8. Can I take metformin while breastfeeding?
Only small amounts enter breast milk, and it is generally considered compatible with breastfeeding a healthy baby. Ask your clinician for individualized advice if your baby is premature, unwell, or has kidney problems.
Related Questions
Related Resources
References
- American Diabetes Association. Management of Diabetes in Pregnancy: Standards of Care in Diabetes—2026.
- National Institute for Health and Care Excellence. Diabetes in pregnancy: management from preconception to the postnatal period (NG3).
- MotherToBaby. Metformin fact sheet.
- NHS. Pregnancy, breastfeeding and fertility while taking metformin.
Medical disclaimer: This information is for education and does not replace care from your obstetrician, endocrinologist, midwife, pharmacist, or other qualified healthcare professional. Pregnancy treatment must be individualized. Seek urgent medical care for severe symptoms, very high or low glucose, ketones, reduced fetal movement, or signs of preeclampsia.