Early Metformin for Gestational Diabetes Mellitus Does Not Alter Exclusive Breastfeeding at 12 Weeks: Secondary Analysis of the EMERGE Randomized Trial

OBJECTIVE To assess whether initiating metformin at 25 weeks' gestation influences exclusive breastfeeding at 12 weeks postpartum in gestational diabetes mellitus (GDM). RESEARCH DESIGN AND METHODS We performed a prespecified secondary analysis of the Effectiveness of Metformin in Addition to Usual Care in the Reduction of Gestational Diabetes Effects (EMERGE) trial, a multicenter, double-blind, placebo-controlled randomized study. A total of 535 pregnant women (510 with newly diagnosed GDM) were allocated 1:1 to metformin (titrated to 2.5 g/day) or placebo alongside usual care. Infant feeding was prospectively documented at birth and at 4 and 12 weeks postpartum. The primary outcome was exclusive breastfeeding at 12 weeks (intention to treat). Multivariable logistic regression adjusted for race, education, smoking, booking BMI, health care access markers, and diagnostic oral glucose tolerance test (OGTT) glucose levels. RESULTS Twelve-week feeding data were available for 476 pregnancies (236 in the metformin group and 240 in the placebo group). Exclusive breastfeeding at 12 weeks was 27% with metformin and 26% with placebo (absolute difference +1.3%; 95% CI –6.3 to 8.7; P = 0.75). The adjusted odds ratio (OR) for metformin versus placebo was 1.24 (95% CI 0.79–1.96; P = 0.36). Feeding patterns shifted from breastfeeding to formula temporally in both groups (P < 0.001), with no treatment-by-time interaction (P = 0.61). Non-Caucasian ethnicity independently predicted exclusive breastfeeding (adjusted OR 2.37; 95% CI 1.27–4.42). CONCLUSIONS Starting metformin treatment at 25 weeks in the GDM group was not associated with differences in exclusive breastfeeding at 12 weeks postpartum. Where metformin was discontinued at delivery, the results support compatibility of antenatal metformin with breastfeeding objectives. Breastfeeding outcomes were more linked to sociodemographic factors than metabolic ones, highlighting the need for culturally appropriate postpartum support.