Objective: The aim of the study was to investigate whether weight, length, BMI (kilograms per meter squared), and head circumference at birth and their postnatal growth are associated with cognitive abilities at 56 months of age among infants born at term.
Patients and methods: Our sample was composed of 1056 Finnish children born at term, (37 to 41 weeks) free of any major impairments. Weight, length, and head circumference were measured at birth and at 5, 20, and 56 months of age, and BMI was calculated. We assessed cognitive abilities by conducting tests of general reasoning, visual-motor integration, verbal competence, and language comprehension at 56 months of age.
Results: Firstly, for every 1 SD lower in weight or BMI at birth, general reasoning and/or visual-motor integration was >1.20 points lower, and for every 1 SD lower in length or head circumference at birth, abilities across all of the cognitive domains were >1.31 points lower. Second, for every 1 SD slower gain in weight or BMI from birth to 5 months, general reasoning and visual-motor integration decreased by >0.97 points; for every 1 SD slower gain in length from 5 to 20 months and from 20 to 56 months, respectively, visual-motor integration, and verbal competence and language comprehension decreased by >1.03 points; and for every 1 SD slower increase in head circumference from birth to 5 months and from 5 to 20 months, respectively, visual-motor integration and language comprehension decreased by >1.17 points. Third, tests for nonlinear relationships revealed that, in some cases, large body size and faster growth were also associated with lower scores in cognitive tests.
Conclusions: Our findings suggest that, even within the range of children born at term, prenatal and postnatal growth in body size are associated with individual differences in cognitive abilities.