Why should metformin be stopped in pregnant women with polycystic ovary syndrome (PCOS) who develop intrauterine growth restriction (IUGR) or pre-eclampsia?

Why Metformin Should Be Stopped in PCOS Patients Who Develop IUGR or Pre-eclampsia

Metformin must be discontinued in pregnant women with PCOS who develop intrauterine growth restriction (IUGR) or pre-eclampsia because it can worsen growth restriction or cause acidosis in the setting of placental insufficiency. 1

Mechanism of Harm in Placental Insufficiency

The American Diabetes Association explicitly states that metformin should not be used in pregnant women with hypertension, preeclampsia, or those at risk for intrauterine growth restriction due to the potential for growth restriction or acidosis when placental insufficiency is present. 1 This is a critical safety concern because:

• Placental transfer is extensive: Metformin readily crosses the placenta, resulting in umbilical cord blood levels equal to or higher than maternal levels 1, 2
• IUGR indicates placental dysfunction: Both IUGR and pre-eclampsia are manifestations of suboptimal uterine-placental perfusion 1
• Metformin accumulation risk: When placental function is compromised, the fetus may be exposed to even higher concentrations of metformin with impaired clearance, potentially leading to lactic acidosis 1, 2

Why Metformin Has No Protective Role

There is no evidence-based need to continue metformin in PCOS patients once pregnancy is confirmed, as it does not prevent the complications that necessitate its discontinuation:

• Does not prevent gestational diabetes: Metformin has not been shown to prevent gestational diabetes even in high-risk women with PCOS, obesity, or insulin resistance 2, 3
• Does not prevent pre-eclampsia: Evidence for metformin preventing pre-eclampsia is inconsistent and insufficient 4
• No benefit for IUGR prevention: Randomized controlled trials have not demonstrated that metformin prevents IUGR in PCOS pregnancies 1, 2

Long-term Offspring Concerns Support Discontinuation

The decision to stop metformin is further supported by concerning long-term metabolic effects in exposed offspring:

• Children exposed to metformin in utero at 7-9 years had higher weight, waist-to-height ratio, and waist circumference compared to insulin-exposed children 1, 2
• Follow-up at 4-10 years showed higher BMI, weight-to-height ratios, and borderline increases in fat mass 1, 2
• Meta-analyses demonstrate metformin exposure results in smaller neonates with accelerated postnatal growth, leading to higher childhood BMI 1, 2

Clinical Algorithm for Metformin Management

When to stop metformin in PCOS pregnancy:

1. Immediately upon diagnosis of pre-eclampsia (hypertension with proteinuria or end-organ dysfunction) 1, 2
2. Immediately upon diagnosis of IUGR (estimated fetal weight <10th percentile for gestational age) 1, 2
3. When hypertension develops (even without full pre-eclampsia criteria) 1, 5
4. When any signs of placental insufficiency appear (abnormal umbilical artery Doppler, oligohydramnios, poor fetal growth velocity) 1

Standard discontinuation timing if no complications:

• Metformin should be discontinued by the end of the first trimester when used solely for PCOS and ovulation induction 2, 3
• Continue only if the patient has type 2 diabetes requiring ongoing treatment 1, 2

Common Pitfalls to Avoid

• Do not continue metformin simply because it helped achieve conception: The preconception benefits do not translate to pregnancy benefits for PCOS patients without diabetes 2
• Do not delay discontinuation when complications arise: The risk of acidosis and worsening growth restriction requires immediate cessation 1, 5
• Do not assume metformin will prevent pregnancy complications: It has not been proven effective for preventing gestational diabetes, pre-eclampsia, or IUGR in PCOS patients 2, 3