Nonhuman primate models of gestational diabetes have produced fetopathies most similar to those of the human infant of the mother with gestational diabetes (IGDM). Fetal hyperglycemia, hyperinsulinemia, macrosomia, selective organomegaly, intrauterine death, and placental hyperplasia are hallmarks of the fetopathy of the IGDM. The chronic infusion of insulin into the fetus of a normal pregnant rhesus monkey results in fetal hyperinsulinemia with normal to low plasma metabolic substrate concentrations. Under these conditions, fetal hyperinsulinemia is sufficient to cause fetal growth and hormone changes observed in the human IGDM. Our studies provide evidence that the soft tissue hyperplasia in the fetal macrosomia syndromes in humans and nonhuman primates in which fetal hyperinsulinemia is observed is the direct result of that chronic in utero hyperinsulinemia.