Diabetes mellitus with its resulting derangement of various metabolic fuels, carbohydrates, amino acids, lipids, and ketones has the potential to adversely affect the developing fetus. Therefore, strict glycemic control in pregnancy has become the standard of care in modern obstetrics. A considerable amount of research has been undertaken into the metabolic changes that occur during pregnancy in both women with insulin-dependent diabetes and gestational diabetes. This paper will review current research in normal and diabetic pregnancies both in the fasting and fed states as well as during episodes of hypoglycemia. In normal pregnancy insulin secretion increases throughout gestation whereas peripheral insulin sensitivity is decreased. Fasting levels of plasma glucose are reduced by approximately 10 per cent during the first trimester. Maternal amino acid levels are also reduced in normal pregnancy, although cholesterol and triglyceride levels are increased, most dramatically in the second trimester. As gestation advances, progressively increasing amounts of insulin antagonistic hormones are secreted by the placenta. This leads to gestational diabetes in 2 to 3 per cent of women who exhibit hyperglycemia despite an increased insulin response to oral glucose as well as an increased insulin/glucagon ratio. In insulin dependent diabetes mellitus, the insulin-deficient state results in fasting and postprandial hyperaminoacidemia, hyperlipidemia, and hyperglycemia. These metabolic changes and the resulting hyperglycemic milieu can lead to fetal macrosomia that will result in maternal and fetal morbidity. Therefore, normalization of these fuels with the use of intensive insulin regimens is the goal of therapy during pregnancy.