Birth weight discordance in multiple gestations: occurrence and outcomes.

This paper reviews the patterns of occurrence, measurement and the effect of birth weight discordance on fetal and neonatal mortality in multiple pregnancies (twins and triplets). Birth weight discordance is fairly common among multiple pregnancies, and about one-quarter of the twin deliveries are affected by a birth weight discordance of 15%, while nearly 5% of twin gestations experience severe discordance (>or= 35%). Factors influencing birth weight discordance are exaggerated in triplet deliveries. Approximately 20% of triplet sets experience a birth weight discordance of 25 - 35% and nearly 10% experience severe forms of discordance. Frequencies of discordant sets at >25% discordance along the range of birth weight deciles show an inverse logarithmic relationship in twins, while the best-fit function in triplets is polynomial. Birth weight discordance is significantly associated with both fetal and neonatal mortality. Neonatal demise among the severely discordant smaller twin is significantly more frequent than in a non-discordant smaller twin. The magnitude of the effect is greater when one or both of the discordant twins are concomitantly small for gestational age.