Continuous Glucose Monitoring (CGM) Substudy of the DECIDE RCT

Inclusion Criteria: * Singleton gestation. Twin reduction to singleton, either spontaneously or therapeutically, is eligible if it occurred before 14 weeks gestational age. * Age \>18 years * Gestational age at randomization between 200/7 - 316/7 weeks based on project gestational age. * GDM diagnosis between 200/7 - 316/7 weeks based on project gestational age. * Requires medication for glucose control defined as ≥30% elevated glucose values (either fasting or postprandial or both) in the week prior to randomization per determination of the provider or documented in the medical record. * Patient willingness and ability to attend 2-year follow-up visit. * Patient willingness to wear and return a blinded CGM device. It is possible that some enrolled individuals may choose to use a separate CGM device for glucose monitoring as part of clinical care. Exclusion Criteria: * Renal disease (serum creatinine \>1.3 mg/dL) due to the potential impact of metformin on renal function. * Major structural malformation of the fetus. * Known fetal aneuploidy based on invasive testing or positive for aneuploidy on cell-free fetal DNA screening. * Contraindication to metformin or insulin, including: history of lactic acidosis, intractable nausea and vomiting, prior documented allergy and/or anaphylaxis. * Pregestational diabetes documented in the medical record, GDM diagnosis \<20 weeks, or prior A1c\>6.5% * Fasting hyperglycemia \>115 mg/dl for ≥50% of fasting glucose values in the past wee…