http://bmj.com/cgi/content/abstract/323/7308/310
BMJ 2001;323:310-314 ( 11 August )
Papers
Influence of variation in birth weight within normal range and within
sibships on IQ at age 7 years: cohort study
Thomas D Matte
a Center for Urban Epidemiologic Studies, New York Academy of
Medicine, 1216 Fif(pvenue, New York, NY 10029, USA, b Department
of Epidemiology, Mailman School of Public Health, Columbia University,
622 West 168th Street, New York, NY 10032, USA, c Division
of Biostatistics, Mailman School of Public Health
Correspondence to: T D Matte tmatte@nyam.org
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 Some evidence suggests the association might also apply to
children of normal birth weight What this study adds 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 |
BMJ 2001
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