Pregnancy causes a multitude of metabolic changes within a woman’s body in order to provide the proper nutrients to the developing fetus. In women with type 1 and type 2 diabetes and gestational diabetes mellitus (GDM), these metabolic perturbations must be treated distinctly and aggressively to optimize fetal development and health. Pregestational diabetes (either type 1 or type 2) has the potential to subject the developing fetus to abnormal maternal glucose levels resulting in problems with organogenesis producing congenital abnormalities or spontaneous abortion. Furthermore, GDM presents after organogenesis in the second part of pregnancy; therefore, the major risk for the fetus is macrosomia. Although the goal for dietary therapy for each of these disorders is the same, which is euglycemia, the means to achieve it are very different and somewhat controversial. In the case of GDM, the mainstay of therapy is medical nutritional therapy, whereas in insulin-requiring diabetes, dietary therapy is compensated with premeal insulin injections. In this chapter, the metabolic changes in normal pregnancy will be presented followed by the general guidelines for pregnancy. Fetal complications associated with inadequate nutrition or metabolic perturbation will be briefly explored, followed by issues and treatment for GDM, with emphasis on specific dietary therapies for GDM.