Are metformin and telmisartan safe for use during pregnancy?

Telmisartan is Contraindicated in Pregnancy; Metformin May Be Acceptable with Important Caveats

Telmisartan must be discontinued immediately when pregnancy is detected or ideally before conception, as it is absolutely contraindicated during pregnancy due to severe fetal harm. 1, 2 Metformin appears acceptable for use during pregnancy in women with gestational or type 2 diabetes, though concerns about offspring outcomes require careful consideration and informed consent. 3, 4, 5

Telmisartan: Absolute Contraindication

Angiotensin receptor blockers (ARBs) like telmisartan are contraindicated throughout pregnancy because they cause serious fetal complications including:

• Fetal renal dysplasia
• Oligohydramnios (reduced amniotic fluid)
• Pulmonary hypoplasia
• Intrauterine growth restriction
• Fetal and neonatal death 1

The FDA drug label explicitly states that telmisartan "can cause fetal harm when administered to a pregnant woman" and that "use of drugs that act on the renin-angiotensin system during the second and third trimesters of pregnancy reduces fetal renal function and increases fetal and neonatal morbidity and death." 2

Critical Timing Considerations

While first-trimester exposure to ACE inhibitors/ARBs may not increase congenital malformations 1, the medication must be stopped as soon as pregnancy is detected to prevent devastating second- and third-trimester fetopathy. 1 For women of childbearing potential taking telmisartan, preconception counseling and immediate medication switching are essential.

Safe Alternatives for Hypertension in Pregnancy

The 2025 ADA guidelines clearly identify safe antihypertensive options 1:

• Methyldopa (first-line)
• Nifedipine (long-acting)
• Labetalol
• Clonidine

Target blood pressure during pregnancy should be 110-135/85 mmHg to balance maternal safety with fetal growth. 1

Metformin: Acceptable with Informed Consent

Metformin can be used during pregnancy for gestational diabetes and type 2 diabetes, but requires thorough discussion of potential offspring effects.

Maternal Benefits

Metformin provides several advantages over insulin monotherapy:

• Comparable glycemic control 4, 5
• Reduced maternal weight gain 4, 5, 6
• Lower insulin requirements in type 2 diabetes 7
• Reduced risk of neonatal hypoglycemia 5, 6
• Possibly reduced macrosomia (large babies) 6, 7
• Better patient acceptability than insulin 6

Safety Profile: Short-Term

No increased risk of congenital malformations has been demonstrated with first-trimester metformin exposure. 3, 4, 6 A 2024 study in Annals of Internal Medicine found that continuing metformin plus insulin versus switching to insulin alone resulted in minimal difference in nonlive births (risk ratio 1.02) and was compatible with anywhere from a 49% decrease to 9% increase in congenital malformations—essentially showing no clear harm. 3

Neonatal outcomes are generally favorable, with no increase in:

• Low APGAR scores
• NICU admissions
• Respiratory distress syndrome 5

Critical Concerns: Offspring Outcomes

The major uncertainty with metformin centers on long-term offspring effects, particularly:

1. Small-for-gestational-age (SGA) births: Multiple studies show increased rates of SGA infants, especially in women with type 2 diabetes who have hypertension or kidney disease 5, 6, 7

2. Childhood obesity and altered body composition: Some follow-up studies demonstrate increased childhood obesity and altered fat distribution in offspring exposed to metformin in utero 4, 5, 6

3. Preterm birth: Metformin has been associated with increased preterm delivery rates, though this finding is inconsistent across studies 5

4. Placental transfer: Metformin crosses the placenta with fetal plasma concentrations comparable to maternal levels at delivery, raising questions about fetal programming effects 4

Clinical Decision Algorithm for Metformin Use

Use metformin when:

• Gestational diabetes or type 2 diabetes requiring pharmacologic treatment
• Normal fetal growth on ultrasound
• No maternal hypertension or kidney disease
• Patient preference after informed consent discussion
• Insulin resistance is a primary concern

Avoid or discontinue metformin if:

• Fetal growth restriction detected on ultrasound 7
• Maternal hypertension present 7
• Maternal kidney disease present 7
• Patient declines after discussion of offspring uncertainty

Essential monitoring:

• Serial ultrasounds for fetal growth assessment
• Blood pressure monitoring throughout pregnancy
• Glycemic control assessment

Informed Consent Discussion Points

When prescribing metformin in pregnancy, explicitly discuss:

• Proven short-term safety regarding birth defects
• Potential for smaller babies (may be beneficial if macrosomia risk, concerning if growth restriction)
• Uncertainty about long-term childhood metabolic effects
• Alternative option of insulin with established long-term safety profile
• Need for ongoing fetal growth monitoring

Common Pitfalls to Avoid

1. Continuing telmisartan into pregnancy: This is never acceptable. Switch to safe alternatives (methyldopa, nifedipine, labetalol) before conception or immediately upon pregnancy detection.

2. Using metformin without fetal growth monitoring: Given SGA concerns, serial ultrasounds are mandatory.

3. Prescribing metformin to women with hypertension or kidney disease: This subgroup appears at higher risk for SGA births. 7

4. Failing to obtain informed consent for metformin: The long-term offspring data remain incomplete, requiring shared decision-making. 4, 8

5. Assuming metformin prevents gestational diabetes: Evidence for prevention is disappointing; metformin is for treatment, not prevention. 7