Although major advancements have been made in improving glycemic management in type 1 diabetes mellitus (DM), women entering pregnancy with type 1 DM continue to be at dramatically increased risk for adverse maternal and neonatal outcomes, including hypertensive disorders of pregnancy (HDP). At present, there is a lack of effective preventive interventions for HDP, which are associated with significant maternal and neonatal morbidity and mortality. Clinical and in vitro data have shown promise for metformin in prevention of HDP in non-diabetic women. Metformin has a reassuring fetal safety profile and has been well studied in type 1 DM outside of pregnancy. The hypothesis to be tested in this application is that compared to usual care, daily oral metformin therapy initiated prior to 20 weeks' gestation in women with type 1 DM reduces the frequency of HDP.

Inclusion Criteria:

• Pregnant women 18-50 years
• 12 0/7 and 19 6/7 weeks of gestation
• Diagnosed with type 1 DM prior to pregnancy.

Exclusion Criteria:

• Known allergy or adverse reaction to metformin
• Multiple gestation
• Abnormal obstetrical ultrasound suspicious for major congenital abnormality, known or suspected fetal aneuploidy
• Medical comorbidities that increase risk for metformin use: renal insufficiency (creatinine > 1.1 mg/dL), proteinuria (P:C >0.3 or 24-hour urine protein > 300 mg), active liver disease (acute hepatitis, chronic active hepatitis, persistently abnormal liver enzymes 3x above normal), inflammatory bowel disease (Crohn's and Ulcerative colitis), major hematologic disorder (including alloimmune and isoimmune thrombocytopenia).

Only pregnant women are eligible to participate in the study