Birth weight in twins: an analysis of discordancy and growth retardation.

Sonographic estimation of fetal weight offers the potential of antenatal diagnosis of discordancy and growth retardation in twins. Although standards for intertwin birth weight percent differences in infants over 2500 g are available, similar norms are not available at lower weights. Intertwin birth weight percent differences in live-born twins above 500 g in 500-g increments were analyzed. Intertwin birth weight differences, expressed as a percentage of the weight of the larger twin, were relatively uniform across a wide range of birth weights. Differences above 15% were increasingly more likely to be associated with growth retardation in one of the infants.