Perinatal Outcomes Associated With Metformin Use During Pregnancy in Women With Pregestational Type 2 Diabetes Mellitus

OBJECTIVE We emulated a modified randomized trial (Metformin in Women With Type 2 Diabetes in Pregnancy [MiTy]) to compare the perinatal outcomes in women continuing versus discontinuing metformin during pregnancy among those with type 2 diabetes treated with metformin plus insulin before pregnancy. RESEARCH DESIGN AND METHODS This study used two health care claims databases (U.S., 2000–2020). Pregnant women age 18–45 years with type 2 diabetes who were treated with metformin plus insulin at conception were eligible. The primary outcome was a composite of preterm birth, birth injury, neonatal respiratory distress, neonatal hypoglycemia, and neonatal intensive care unit admission. Secondary outcomes included the components of the primary composite outcome, gestational hypertension, preeclampsia, maternal hypoglycemia, cesarean delivery, infants large for gestational age, infants small for gestational age (SGA), sepsis, and hyperbilirubinemia. We adjusted for potential baseline confounders, including demographic characteristics, comorbidities, and proxies for diabetes progression. RESULTS Of 2,983 eligible patients, 72% discontinued use of metformin during pregnancy. The average age at conception was 32 years, and the prevalence of several comorbidities was higher among continuers. The risk of the composite outcome was 46% for continuers and 48% for discontinuers. The adjusted risk ratio was 0.92 (95% CI 0.81, 1.03). Risks were similar between treatments and consistent between databases for most secondary outcomes, except for SGA, which was elevated in continuers only in the commercially insured population. CONCLUSIONS Our findings were consistent with those reported in the MiTy randomized trial. Continuing metformin during pregnancy was not associated with increased risk of a neonatal composite adverse outcome. However, a possible metformin-associated risk of SGA warrants further consideration.