Growth discordance and the effect of a male twin on birth weight of its female co-twin: a population-based study.

There is an ongoing controversy whether the presence of a male twin affects the birth weight of its female co-twin, and if affected, whether birth weights of females are lower or higher in the presence of a male co-twin. The purpose of this study was to study this controversy in a large population of 12,639 twin pairs born in Israel between 1993-1998. Comparison of 7566 females in same-sex (FF) pairs with 4900 females in unlike-sex (FM) pairs, adjusted for the lighter and heavier twin in the pair, showed significantly higher birth weights of females in FM pairs. The same trend was found when concordant pairs and discordant pairs (birth weight difference > 25%) were compared separately. No significant difference was found when 7912 males in same-sex (MM) pairs were compared with 4900 males in FM pairs, adjusted for the lighter and heavier sib. Our study supports a definite intrauterine effect of the male twin on birth weight of its female co-twin. This consequence may potentially be associated with fetal programming of adult disease.