Gestational diabetes (GDM) represents a significant and growing source nationwide of morbidity and mortality for both mothers and infants. Between 5 and 10% of pregnancies are complicated by GDM. Infants exposed in utero to hyperglycemia from GDM have an increased risk of neonatal complications as well as an increased prevalence of a number of chronic diseases, including type 2 diabetes and schizophrenia. Recent research shows that treatment with metformin begun as soon as hyperglycemia is detected in pregnancy is safe and effective. Additional studies demonstrated that metformin is able to decrease the incidence of type 2 diabetes among individuals at risk for this disease. Based upon these findings, the investigators propose to test the hypothesis that metformin therapy for women at risk for gestational diabetes, started prior to conception and before the onset of hyperglycemia, both decreases the incidence of and improves the maternal and neonatal outcomes from GDM.