Twenty-five discordant twin pairs were assessed ultrasonically in a longitudinal fashion and were compared with a group of 60 concordant twin pairs. The growth parameters of the larger fetus of the discordant pair did not differ significantly from the concordant twins, while the smaller of the discordant pair exhibited a slower rate of intrauterine growth as early as 23 to 24 weeks. We conclude that (1) twins who ultimately become discordant exhibit demonstrable differences as early as 23 to 24 weeks; (2) the smaller twin in a discordant pair has a much slower rate of growth between 33 to 37 weeks; and (3) using estimated fetal weight to predict concordancy and discordancy by Shepard's and Hadlock's tables are equally efficacious, although Hadlock's table can be used more often because biparietal diameter cannot be obtained as often as femur length.