In normal and diabetic pregnancies, the placenta functions as a complex endocrine gland that modulates all classes of maternal nutrients to the fetus. The metabolic alterations of normal pregnancy are diabetogenic and associated with modest resistance to endogenous insulin. Pregnant women with carbohydrate intolerance represent three metabolically heterogeneous groups: type I (insulin-dependent), type II (non-insulin-dependent), and gestational diabetes. Patients with type I diabetes are at risk for ketosis and require replacement therapy because of a deficient production of insulin. They have decreased 24-hour, around-the-clock levels of C-peptide and glucagon, and lower nocturnal cortisol values and higher 24-hour prolactin levels than those of women with type II diabetes. Type II pregnant diabetic patients are not prone to ketosis and are more resistant to endogenous and exogenous insulin. They have higher fasting and meal-stimulated levels of C-peptide, accentuated fasting hypertriglyceridemia, and significantly lower high-density lipoprotein cholesterol levels than those of normal or type I women. In gestational diabetes, the metabolic stress of pregnancy evokes reversible hyperglycemia which may be associated with either a surfeit or a deficiency of insulin. These metabolic differences among diabetic pregnant women could have implications for placental structure and function that might influence fetal growth.