Within pair association between birth weight and blood pressure at age 8 in twins from a cohort study.

OBJECTIVES To study the association between birth weight and blood pressure in children from multiple pregnancies (multiplets), mostly twins, to determine whether maternal or genetic factors are responsible for the association. DESIGN Cohort study. SETTING Southern Tasmania. SUBJECTS 888 children including 104 multiplets (32 monozygotic, 72 dizygotic). MAIN OUTCOME MEASURE Systolic blood pressure (mm Hg). RESULTS Blood pressure decreased with birth weight and increased with current body mass. After adjustment for age and body mass, systolic blood pressure changed by -1.94 mm Hg (95% confidence interval -2.89 to -0.98) per 1 kg increase in birth weight of singletons. For multiplets, blood pressure changed by -7.0 mm Hg (-10.1 to -3.9) for each 1 kg increase in birth weight. This was little altered in within pair analyses (-5.3, -13.8 to 3.2) and was similar for both monozygotic (-6.5, -22.5 to 9.4) and dizygotic (-4.9, -15.8 to 6.0) pairs. CONCLUSION Because the association between birth weight and blood pressure was largely unchanged in within pair analyses, exposures originating in the mother (such as nutritional status) cannot be wholly responsible. The association also remained within monozygotic pairs, suggesting that genetic predisposition is not wholly responsible either. The principal causal pathway must concern mechanisms within the fetoplacental unit. The stronger association in multiplets suggests that factors adversely influencing both blood pressure and birth weight are more prevalent in multiple pregnancies.