Technical Abstract: Fetal energy demands are met from the oxidation of maternally supplied glucose and amino acids. During the fasted state, the glucose supply is thought to be met by gluconeogenesis. Underweight women with low body mass index (BMI) might be unable to adequately supply amino acids to satisfy the demands of gluconeogenesis. Glucose kinetics were measured during the first and second trimesters of pregnancy in 10 low-BMI and 10 normal-BMI pregnant women at the 12th hour of an overnight fast using a primed 6 h U-13C glucose infusion and was correlated to maternal dietary and anthropometric variables and birth weight. Low-BMI mothers consumed more energy, carbohydrates, and protein, had faster glucose production (R a) and oxidation rates in the first trimester. In the same trimester, dietary energy and carbohydrate correlated with glucose production, glycogenolysis, and glucose oxidation in all women. Both groups had similar rates of gluconeogenesis in the first and second trimesters. Glucose R a in the second trimester was weakly correlated with the birth weight (r=0.4, P=0.07). Maternal energy and carbohydrate intakes, not BMI, appear to influence glucose R a and oxidation in early and mid pregnancy.