Objective. Given the adverse neurobiological effects of suboptimal nutrition on the developing brain, it is of social and medical importance to determine whether the global prevalence of poor intra-uterine growth causes lasting cognitive deficits. We examined whether sub-optimal intra-uterine growth relates to impaired cognitive outcome by comparing birthweight and cognition in monozygotic (MZ) twins. Our study considered whether children who do not reach their potential birthweight, as indexed by the weight of their heavier twin, also do not attain their potential IQ.
Methods. 71 MZ twin pairs participated (7 years 11 months to 17 years 3 months). The Wechsler Intelligence Scale for Children Third Edition (WISC-III) was administered and Verbal IQ (VIQ) and Performance IQ (PIQ) scores calculated. Regression was used to relate within pair differences in birthweight to within pair differences in IQ scores.
Results. VIQ but not PIQ score was affected by pre-natal growth restriction. The results suggest that the mean advantage for heavier twins relative to their lighter co-twins can be as much as half a SD in VIQ points. In pairs with minimal birthweight discordance, heavier twins had lower VIQ scores than their lighter co-twins
Conclusions. Our study suggests that lower birthweight can negatively impact on cognition in the long term, not only in infants born small, but across the birthweight spectrum. Restricting analyses to MZ twins enables the effect of reduced intra-uterine growth on cognition to be examined independently of confounding factors including parental IQ and education, gender, age, genes, and gestation
