The Association of High Birth Weight With Intelligence in Young Adulthood: A Cohort Study of Male Siblings

Abstract

We aimed to explore why, in population studies, the positive association between normal-range birth weight and intelligence becomes negative at the highest birth weights. The study population comprised 217,746 Norwegian male singletons born at term between 1967 and 1976. All had data on birth weight and intelligence quotient (IQ) score at the time of military conscription; 137,574 had data on sibling birth weights; and 62,906 had data on male sibling birth weights. We estimated associations between birth weight and IQ score by ordinary least squares regression for the total study population and by fixed-effects regression for comparisons of brothers. The crude mean IQ score was 1.2 points (95% confidence interval (CI): 0.3, 2.2) lower for those with birth weights of 5,000 g or more compared with the reference group (with birth weights of 4,000-4,499 g). This difference leveled off to 0.0 (95% CI: -0.8, 0.9) in multivariable ordinary least squares regression and reversed to 2.2 points (95% CI: 0.3, 4.2) higher in fixed-effects regression. Results differed mainly because, at a given birth weight, participants who had a sibling with macrosomia had a lower mean IQ score. Nevertheless, within families with 1 or more macrosomic siblings, as in other families, men with higher birth weights tended to have higher IQ scores. Thus, a family-level confounder introduces a cross-level bias that cannot be detected in individual-level studies. We suggest ways in which future studies might elucidate the nature of this confounder.

What birthweight percentile is associated with optimal perinatal mortality and childhood education outcomes?

“…In comparison, most non-Aboriginal fetuses likely grew in a more conducive environment such that the prevalence of asphyxia, birth trauma, and unmeasured family-level factors in the largest infants may have mitigated the neurocognitive benefit of increasing birthweight percentile. 32 Furthermore, our findings ajog.org OBSTETRICS Original Research provide some evidence that the range of birthweight percentile that is optimal for short-term survival is different to that which is optimal for academic outcomes, particularly in Aboriginal infants.…”

High birth weight and perinatal mortality among siblings: A register based study in Norway, 1967-2011

“…We intended to study the entire birth weight distribution, with a particular emphasis on macrosomia and the hook of the inverted J. The a priori hypothesis was that the higher PNM in the macrosomic segment was confounded by unmeasured, shared family factors, as found earlier for macrosomia in association with lower intelligence [ 46 ]. We also wanted to estimate how the participant’s deviation from mean sibling birth weight influenced the birth weight-mortality association.…”

A MicroRNA Signature in Gestational Diabetes Mellitus Associated with Risk of Macrosomia

“…A macrosomic foetus is defined as weighing more than 4 kg. Moreover, macrosomic babies are prone to developing childhood obesity and type 2 diabetes and/or cardiovascular diseases in the later stage of life [1][2][3].…”