Diabetes mellitus describes diseases of abnormal glucose metabolism associated with insulin secretion impairment and/or peripheral insulin resistance. In pregnancy, these disorders are classified into two categories: pregestational (pre-existing type 1 or type 2 diabetes) or gestational disease. Diagnostic criteria for subtypes of disease vary, with recommendations to screen average-risk pregnant patients at 28 weeks. Providers and patients should be cognizant of maternal and neonatal complications of these disorders, such as hypertension, ketoacidosis, hypoglycemia, congenital malformations, macrosomia, cesarean delivery, and long-term increased risk for continued glucose intolerance. Antepartum and intrapartum glucose monitoring, treatment with lifestyle changes and/or insulin, fetal testing, and ultimately delivery timing and method vary based on the patient’s diagnosis and clinical course.