Forty-three consecutive twin pregnancies were evaluated by ultrasound to establish criteria for antenatal detection of discordant fetal growth. For each fetus an attempt was made to measure the biparietal diameter (BPD), abdominal circumference, and femur length; the estimated fetal weight was also calculated based on published formulas. The intrapair differences in BPD, abdominal circumference, femur length, and estimated fetal weight were evaluated as predictors of discordant fetal growth. Although the intrapair difference in BPD measurement was not a statistically significant predictor, an intrapair difference in abdominal circumference of 20 mm or more was found to have sensitivity 80%, specificity 85%, positive predictive value 62%, and negative predictive value 93%. Intrapair difference in the estimated fetal weight was found to be the best predictor of discordant fetal growth (sensitivity 80%, specificity 93%, positive predictive value 80%, and negative predictive value 93%). These data suggest that the intrapair difference in abdominal circumference measurement could be effectively used as a screening test for the diagnosis of discordant fetal growth. If the intrapair difference in abdominal circumference is 20 mm or greater, further evaluation, by determining the intrapair difference in estimated fetal weight, is indicated.