Serial ultrasound examinations were performed during the third trimester in 79 pregnant women with diabetes to establish the onset of accelerated fetal growth. At least three ultrasound examinations were performed, with a minimum scan interval of 2 weeks. Growth curves constructed for femur length and head circumference were similar for fetuses appropriate for gestational age (n = 48) and fetuses large for gestational age (n = 31). The mean changes in femur length and head circumference (expressed as centimeters per week during the early and late third trimesters) did not differ statistically between these two groups. Abdominal circumference growth was clearly accelerated at 32 weeks' gestation in the large for gestational age group (mean +/- SD, 1.36 +/- 0.16 cm/wk) compared with the appropriate for gestational age group (0.901 +/- 0.21 cm/wk, p less than 0.001). With use of a receiver operator characteristic curve, a change in abdominal circumference of 1.2 cm/wk over the period of 32 to 39 weeks' gestation was determined to be an optimal cutoff for detecting excessive fetal growth (sensitivity 84%, specificity 85%). A change in abdominal circumference 1.2 cm/wk was present in 4/4 large-for-gestational age fetuses (less than 4000 gm), in 17/21 (81%) of fetuses with birth weights 4000 to 4499 gm, and in 5/6 (83%) whose weight exceeded 4500 gm. It appears that improved detection of the fetus large for gestational age in diabetic pregnancies may be accomplished by the use of serial ultrasonography during the third trimester.