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 intelligence at age 7 years in children within the normal range of birth weight and 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 years 1959 through 1966. The sample was restricted to children born at >/=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 across the range of birth weight in a linear regression analysis of one randomly selected sibling per family (n= 1683) with adjustment for maternal age, race, education, socioeconomic status, and birth order. Within same sex sibling pairs, differences in birth weight were directly associated with differences 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 differences in birth order, maternal smoking, and head circumference were adjusted for and in an analysis restricted to children with birth weight >/= 2500 g. CONCLUSION The increase in childhood IQ with birth weight continues well into the normal birth weight range. For boys this relation holds within same sex sibships and therefore cannot be explained by confounding from family social environment.