Head circumference at birth and childhood developmental disorders in a nationwide cohort in Denmark

Early markers of Attention Deficit Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD) may improve the understanding and early recognition of these disorders. We aimed to estimate the association between head circumference at birth, a measure of cerebral size at birth, and the risk of ADHD and ASD.

We present a register-based cohort study of all Danish singletons born alive between 1997 and 2013. Cox proportional hazards regression was used for the statistical analyses. Sibling-matched analyses were performed to account for unmeasured confounding shared by siblings.

The analyses included 986 909 new-borns. Compared to normocephalic children, microcephaly was associated with an increased risk of ADHD (hazard ratio [HR] 1.22, 95% confidence interval [CI] 1.12, 1.32). Macrocephaly was associated with a decreased risk of ADHD (HR 0.90, 95% CI 0.82, 0.99). Neither microcephaly nor macrocephaly were associated with ASD (HR 1.06, 95% CI 0.94, 1.19 and 1.03, 95% CI 0.90, 1.19). The largest difference was found within the normocephalic children. A head circumference at the lower limit of normocephaly compared to a head circumference at the upper limit was associated with an increased risk of ADHD (HR 1.52, 95% CI 1.43, 1.63). The sibling analyses confirmed the increased risk of ADHD with decreasing head circumference in the normocephalic children. No other clear associations were present in the sibling analyses.

Within normocephalic children, smaller head circumference at birth was associated with a higher risk of ADHD. Restricted foetal brain growth may be a risk factor for the development of ADHD but not ASD.