An intergenerational study of birthweight: investigating the birth order effect.

In a two-generation study of the nationwide 1958 British cohort and their offspring, we investigated the intergenerational influence of birth order on birthweight. Despite increases in own birthweight by birth order and a positive parent-offspring correlation in birthweights, there was a suggestion that parental birth order was inversely associated with offspring birthweight. This paradoxical finding was due to differential intergenerational birthweight associations, with a weaker association in later-born compared with first-born parents. Our findings suggest that underlying causes of a given birthweight should be taken into account in studies that investigate long term health outcomes.