Traumatic morbidity in the newborn of a diabetic mother occurs in 3 to 9% of vaginal deliveries in diabetic pregnancies. Prediction of diabetic macrosomia by ultrasound measurement of chest diameter and biparietal diameter is evaluated in this study. A macrosomia index was calculated for 70 diabetic pregnancies by subtracting the biparietal diameter from the chest diameter (chest — biparietal diameter). Twenty-three macrosomic infants (weight greater than 4000 g) were delivered. In this study 20/23 (87%) of the infants weighing greater than 4000 g had a chest — biparietal diameter of 1.4 cm or greater. There were 4 cases of shoulder dystocia in 15 patients delivered vaginally. In this study, cesarean section for all fetuses with a chest — biparietal diameter of 1.4 cm or greater would reduce the incidence of traumatic morbidity from 27% to 9%.