4.5. Birth weight
Against previous findings by Sommerfelt et al. (1995), we found that birth weight was not significantly more associated with nonverbal than with verbal skills, although birth weight was significantly associated to the latent variable nonverbal skills only. Our study may have lacked power to detect this differential effect; indeed, the ratio of the two regression parameters was relatively high (nonverbal/verbal = 2.0), suggesting a substantial differential effect. By what mechanism might birth weight have differential cognitive effects? One possibility is for instance that neurotrophic stress may primarily affect later maturing brain regions (e.g. the frontal lobes). However, it is also important to consider that birth weight may not have a causal effect on cognitive measures and may rather be considered as a proxy for other factors such as nicotine consumption during pregnancy or maternal nutrition during pregnancy. Future studies conducted on larger samples likely to include greater number of children with low birth weight (solely 5.5% of the children had a low birth weight (< 2.5 kg) in our study) are needed to confirm our findings and determine how birth weight may impact differentially verbal and nonverbal skills. In conclusion, by using a latent variable approach which allows testing for differential effect of specific factor on specific cognitive skills in a large mother-child cohort, we have provided evidence that, although many broad environmental factors exert broad effects on both verbal and nonverbal skills in children aged 5–6 years, some factors (home cognitive stimulation, breastfeeding duration, and number of older siblings) show differential effects, i.e., greater effects on verbalthan on nonverbal skills.
