Biphasic effects of maternal metabolism on fetal growth. Quintessential expression of fuel-mediated teratogenesis.

More than a decade ago, Norbert Freinkel postulated that alterations in the maternal metabolic milieu at any time during gestation can influence intrauterine development and also may have long-term consequences for certain tissues such as adipocytes, myocytes, pancreatic beta-cells, and neurons. This review illustrates that metabolic alterations early in gestation, such as those that occur in diabetes mellitus, may impair growth of the embryo and increase the risk of dysmorphogenesis. Such delayed growth of the embryo may in turn influence size at birth. In midgestation, metabolic perturbations may accelerate functional maturation of fetal pancreatic beta-cells. Fetal beta-cell development is very sensitive to alterations in the nutrient milieu and may be enhanced in gestational diabetes mellitus (GDM) with only minimal elevations of plasma glucose and minor alterations in other nutrient fuels, including insulinogenic amino acids. Data are reviewed that suggest that the ensuing fetal hyperinsulinemia may promote the development of macrosomia even if metabolic control is satisfactory during late gestation. The overall potential influences of metabolic alterations on intrauterine growth are different in pregnancies complicated by diabetes mellitus throughout gestation (pregestational) and GDM. However, the implications in an individual pregnancy may be defined by the degree of metabolic control at the specific stages of gestation when growth of the embryo, development of fetal beta-cell function, and growth of insulin-sensitive tissues are most critically influenced by the metabolic milieu.