Approximately 1.3% of pregnancies in the USA occur in women with preexisting diabetes mellitus. Women with pregestational diabetes mellitus who are in poor glycemic control during the first 7 weeks of pregnancy when fetal organogenesis is occurring have an increased incidence of spontaneous abortion and fetuses with congenital anomalies. Prepregnancy care of women with either type 1 diabetes mellitus (DM1) or type 2 diabetes mellitus (DM2) must include achievement of glucose control prior to conception to reduce the risk of maternal and fetal complications. Retinopathy, nephropathy, hypertension, and neuropathy each have an impact on the success of the pregnancy. Medical treatment of diabetes during pregnancy and treatment of coexisting diabetic complications will improve maternal and fetal outcome of pregnancies complicated by preexisting diabetes mellitus.