Background: Intra-uterine growth is a powerful predictor of infant mortality and of health, developmental and socioeconomic outcomes in adulthood. The question about whether this relationship is causal rather than driven by unobserved characteristics of low-weight infants is, however, still open. We use twin models to examine the hypothesis that in utero growth has a detrimental impact on cognitive development in childhood.
Methods: We merge birth registry information on birthweight with standardized Math and Spanish test scores for all fourth graders in Chile to create a prospective data set. Twin fixed-effects models are used to estimate the causal effect of intra-uterine growth on test scores. Fixed-effect estimates are compared with traditional regression results in a cross-section of births to gauge the omitted variable bias emerging from unobserved genetic, maternal and pregnancy-related factors in cross-sectional models.
Results: Birthweight differences within twin pairs have a substantial effect on test scores. A 400-g increase in birthweight results in a 15% standard deviation increase in Math scores. The effect is larger among (estimated) monozygotic than dizygotic pairs, reaching >20% standard deviation. The effect varies across family socioeconomic status. It is strong among disadvantaged families but it nearly disappears among advantaged ones.
Conclusion: Scarcity of uterine resources resulting in intra-uterine growth restriction has a detrimental effect on cognitive development in childhood. This effect interacts with family socioeconomic status (SES), so that low-SES families reinforce the effect of low birthweight and high-SES families fully compensate for it. Findings are particularly relevant in the developing world, where intra-uterine growth restriction is the main determinant of low birthweight.