Pregestational diabetes mellitus represents one of the most challenging medical complications of pregnancy because of the need for frequent monitoring and adjustment of medications as well as the potential for maternal and fetal complications. This document provides an overview of the current understanding of pregestational diabetes mellitus and suggests management guidelines during pregnancy. Because few well-designed studies have been performed, many of the guidelines are based on expert and consensus opinion. This document has been updated to reflect current data on pregestational diabetes. This Practice Bulletin is updated with summary information to counsel and manage women with pregestational diabetes before and during pregnancy, more recent literature reflecting experience with continuous subcutaneous insulin infusion during pregnancy, an expanded section on the role of oral hypoglycemic agents in pregnancy, and the option of long-acting reversible contraception during the postpartum period.