The influence of a male twin on birthweight of its female co-twin - a population-based study.

Fetal growth is influenced by various determinants, with males being heavier than females. It was recently suggested that female birthweight tends to be increased by the presence of a male co-twin. We evaluated this hypothesis in the Israeli population. We reviewed a retrospective population database of the 1993-1998 Israeli Birth Registry. We compared male birthweight in the male-male and male-female combination sets and female birthweight in the female-female and male-female combination sets. We compared the mean birthweight of males and females in three groups of total twin birthweight. The sample of 12,686 sets included 50.6% males. The male-male combination was found in 31.3% sets, female-female in 38.7% and unlike-sex combination in 30.0%. A total twin birthweight of less than 3000g was found in 7.2% sets, of 3001-5000g in 50.7% sets, and of over 5000g in 42.1% sets. The mean male birthweight was 2398 +/- 602 g and the mean female birthweight was 2316 +/- 566g, p <.0001 (95% CI of 67, 96). The mean birthweight of males in the male-male combination was significantly lower than in the unlike-sex combination, p <.0001, in total birthweights over 3000g. No significant difference was found between the mean birthweight of females in the female-female and in the unlike-sex combinations in all total birthweight strata. Our findings do not substantiate a male-related inter-twin influence on female birthweight. Population differences might underlie the opposing observations in the literature.