Several maternal plasma fuel abnormalities have been described in gestational diabetes mellitus (GDM), and all may contribute to the development of fetal macrosomia, generally because of the surfeit of calories they provide. Elevated maternal plasma glucose and amino acid concentrations represent key disturbances, because they are also well-known fetal pancreatic β-cell secretagogues. Fetal hyperinsulinemia contributes to macrosomia in a special way by selectively accelerating fuel utilization and storage in insulin-sensitive fetal tissues. Maternal obesity intensifies the insulin resistance already present in late pregnancy and probably exaggerates the metabolic abnormalities attending GDM that impact on fetal growth and development. However, the means by which maternal obesity per se promotes the development of heavy babies in nondiabetic pregnancies remains poorly defined. Significant correlations exist between newborn birth weight and the levels of maternal plasma glucose, amino acids, free fatty acids, and triglycerides in diabetic pregnancies. However, the relative influence of each disturbance on fetal birth weight remains controversial and requires more detailed investigation.

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