Birth month and infant gross motor development: Results from the Japan Environment and Children’s Study (JECS)

This is the authors' abstract. We don't add key points for this paper.

The association between birth month and neurodevelopmental or psychiatric disorders has been investigated in a number of previous studies; however, the results have been inconsistent. This study investigated the association between birth month and child gross motor development at 6 and 12 months of age in a large cohort of infants (n = 72,203) participating in the Japan Environment and Children's Study (JECS). Gross motor development was assessed using the Ages and Stages Questionnaire (ASQ-3). At 6 months and 12 months, 20.7% and 14.2%, respectively, had ASQ-3 indications of gross motor problems. Birth month was strongly associated with gross motor development at both time points, particularly at 6 months. Summer-born infants had the worst outcomes at both 6 months and 12 months of age. This outcome applied to the ASQ-3 score itself and to the adjusted Relative Risk (aRR), with the highest aRRs (relative to January-born) among August-born (aRR 2.51; 95%CI 2.27-2.78 at 6 months), and June-born (aRR 1.84; 95%CI 1.63-2.09 at 12 months). Boys had better scores than girls both at 6 and 12 months of age. We speculate that seasonal factors-such as maternal vitamin D deficiency and influenza infection-affecting the fetus in early pregnancy might account for the findings.