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 week (due to the high risk of metformin failure with fasting hyperglycemia).
• * Enrolled in a trial that influences primary study outcomes of the parent DECIDE trial (composite neonatal outcome at delivery or childhood body mass index at 2 years).
• * Prenatal care or delivery planned at a location where access to the complete electronic medical record will not be available to research staff.
• * Language barrier (appropriate translation resources unavailable at the site).
• * Participation in this trial in a previous pregnancy. Patients who were screened in a previous pregnancy, but not randomized, may be included.
• * In addition, individuals who report a prior allergy or sensitivity to CGM will also be excluded.