The currently accepted definition of gestational diabetes mellitus (GDM) is rather broad. One might expect that fetal and neonatal complications that may occur in GDM pregnancy would be similar to those in pregestational diabetic pregnancy. Comparative evaluation of reported data on morbidity in GDM are often hampered by confounding variables (maternal age, parity, obesity) as well as the influence of factors such as ethnic origin, diagnostic criteria, and intervention during pregnancy. Recent observations indicate that GDM may be associated with increased incidence of fetal malformation and perinatal mortality. Such poor outcome is likely confined to a subset of GDM patients in whom diabetes was present but unrecognized before pregnancy. The most frequent and significant morbidity is fetal macrosomia, which in turn is associated with increased risk of birth injuries and asphyxia. In a nationwide study in Sweden (1991-1993) of a large series (n = 3.322) of treated GDM pregnancies, perinatal mortality rate was not increased; but the rate of preeclampsia was doubled, and the rate of emergency cesarean section was 1.6 times higher than in the background population. The rates of fetal macrosomia (> or = 4,500 g), asphyxia, and transient tachypnea were two to three times higher than normal Erb's palsy was 0.7 and 5% in vaginally delivered infants weighing < 4,500 and > or = 4,500 g, respectively. There is a clear need to define the various levels of glucose intolerance in the mother that may have an adverse effect on the offspring. Of equal importance is to standardize and systematize the criteria used to assess the significance of any such impact.