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

BMJ 2001; 323 doi: (Published 11 August 2001) Cite this as: BMJ 2001;323:310

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  1. Thomas D Matte (tmatte{at}, senior epidemiologista,
  2. Michaeline Bresnahan, assistant professorb,
  3. Melissa D Begg, associate professorc,
  4. Ezra Susser, professorb
  1. a Center for Urban Epidemiologic Studies, New York Academy of Medicine, 1216 Fifth Avenue, New York, NY 10029, USA
  2. b Department of Epidemiology, Mailman School of Public Health, Columbia University, 622 West 168th Street, New York, NY 10032, USA
  3. c Division of Biostatistics, Mailman School of Public Health
  1. Correspondence to: T D Matte
  • Accepted 3 May 2001


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.

What is already known on this topic

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

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


  • Funding New York Community Trust.

  • Competing interests None declared.

  • Accepted 3 May 2001
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