Metformin
1. Pregnancy Evidence & Regulatory Labeling
FDA Pregnancy and Lactation Labeling Rule (PLLR 8.1) & EMA SmPC Section 4.6
Clinical Risk Summary
Metformin is widely used for gestational diabetes mellitus (GDM) and pre-existing type 2 diabetes in pregnancy when insulin is unavailable or not tolerated. Extensive data from randomized controlled trials (such as the MiG trial) and large registry cohorts show no increased risk of major birth defects, miscarriage, or perinatal mortality.
Human Pregnancy Experience & Clinical Studies
Meta-analyses of >4,000 first-trimester exposed pregnancies demonstrated no elevated risk of congenital organ malformations compared with insulin-treated controls.
Animal Reproductive & Developmental Toxicity Data
Animal studies in rats and rabbits up to 2-4 times human therapeutic exposure showed no adverse developmental effects.
Clinical Considerations for Prescribers
Insulin remains the preferred first-line agent for gestational diabetes according to ACOG and ADA guidelines; however, metformin is an acceptable alternative. Metformin cross-placental transfer is substantial, so maternal glycemic control must be strictly monitored.
Placental Transfer
Crosses the placenta readily; cord blood concentrations are comparable to or slightly higher than maternal plasma.
2. Lactation & Breastfeeding Safety
NIH / NLM LactMed® Database & FDA Labeling Section 8.2
Summary of Use during Lactation
Metformin is considered compatible with breastfeeding by NIH LactMed and the AAP. Although excreted into breast milk, the amount ingested by the infant is low (Relative Infant Dose ~0.28% to 1.08%), and infant plasma concentrations are negligible.
Drug Levels in Breast Milk & Relative Infant Dose
Mean milk concentration ranges from 0.13 to 0.41 mg/L with a milk-to-plasma ratio of approximately 0.35 to 0.63.
Effects in Breastfed Infants & Monitored Signs
No adverse clinical reactions or hypoglycemia observed in breastfed infants.
Effects on Lactation & Milk Production
May improve milk production in women with polycystic ovary syndrome (PCOS) experiencing low milk supply.
Alternate Medications with Established Lactation Data
4. Regulatory Source Provenance & Comparison
Direct attribution to official regulatory documents without synthesized consensus
| Regulatory Organization | Jurisdiction | Document / Framework | Effective Date | Official Source Link |
|---|---|---|---|---|
|
National Institutes of Health (NIH) / NLM
LactMed® Database (Drugs and Lactation)
|
United States / Global | evidence_monograph | 2026-08-01 | Official Record |