Influence of variation in birth weight within normal range and within sibships on IQ at age 7 years: cohort study

Objective To examine the relation between birth weight and measured intelli gence at age 7 years in children within the normal range of birth weight an d in siblings. Design Cohort study of siblings of the same sex. Setting 12 cities in the United States. Subjects 3484 children of 1683 mothers in a birth cohort study during the y ears 1959 through 1966. The sample was restricted to children born at great er than or equal to 37 weeks gestation and with birth weights of 1500-3999 g. Main outcome measure Full scale IQ at age 7 years. Results Mean IQ increased monotonically with birth weight in both sexes acr oss the range of birth weight in a linear regression analysis of one random ly selected sibling per family (n = 1683) with adjustment for maternal age, race, education, socioeconomic status, and birth order. Within same sex si bling pairs, differences in birth weight were directly associated with diff erences in IQ in boys (812 pairs, predicted IQ difference per 100 g change in birth weight = 0.50, 95% confidence interval 0.28 to 0.71) but not girls (871 pairs, 0.10, - 0.09 to 0.30). The effect in boys remained after diffe rences in birth order, maternal smoking, and head circumference were adjust ed for and in an analysis restricted to children with birth weight greater than or equal to 2500 g. Conclusion The increase in childhood IQ with birth weight continues well in to the normal birth weight range. For boys this relation holds within same sex sibships and therefore cannot be explained by confounding from family s ocial environment.