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.
Cohort study of siblings of the same sex.
12 cities in the United States.
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 1500-3999g.
Main outcome measure
Full scale IQ at age 7 years.
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.
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.
What is already known on this topic
IQ at school age is linked to birth weight among low birthweight babies
Some evidence suggests the association might also apply to children of normal birth weight
What this study adds
IQ at age 7 years is linearly related to birth weight among children of normal birth weight
The relation was not due to confounding by maternal or socioeconomic factors
IQ is also associated with differences in birth weight between boy sibling pairs but not girls