Profiles of hemoglobin A1c (HbA1c) and concentrations of plasma glucose and 18 plasma amino acids were obtained in ten nonobese, insulin-dependent type I diabetic women, in 9 age- and weight-matched normal women and in ten obese nondiabetic women throughout pregnancy and postpartum. In late gestation, the period of maximum fetal growth, average HbA1c, plasma glucose, and total amino acid concentrations in diabetic mothers were significantly elevated above lean control values. No differences existed between the obese and lean control groups. Lean diabetic mothers also had significantly heavier babies (mean +/- SEM) relative to the 50th percentile for gestational age and sex (119 +/- 9%) than did the lean control group (94 +/- 3%, P less than .05). Relative birth weights among control lean and obese mothers did not differ significantly (94 +/- 3% v 104 +/- 5%). Late pregnancy profiles of HbA1c and average plasma glucose did not correlate with relative weight of neonates whereas average total plasma amino acids and six individual amino acids did correlate with this parameter. These data suggest that maternal plasma amino acid concentrations may influence fetal weight generally and may have an important role in the development of fetal macrosomia in diabetic pregnancies.