Birth Seasonality and Risk of Autism Spectrum Disorder

Background: Season of birth has been hypothesized to be a risk factor for autism spectrum disorder (ASD). However, the evidence has been mixed and limited due to methodological challenges.

Objectives: Examine birth and ASD diagnosis data across multiple countries for statistical evidence of birth seasonality in ASD.

Methods: We examine ASD birth trends for 9,560,874 births across 5 countries. ASD birth prevalence data were obtained from the International Collaboration for Autism Registry Epidemiology database, including children born in Denmark, Finland, Norway, Sweden, and Western Australia. Parametric and non-parametric methods including empirical mode decomposition and logistic regression were used to assess seasonality.

Results: We demonstrate seasonal variation in ASD births for the countries of Denmark, Finland, and Sweden, including a small increase in risk for children born in the fall (i.e., conceived in the winter). For example, for Sweden in the months of January, March, April, and June, the observed ASD rate was approximately 4 cases/10,000 births lower than would be expected, while for the months of September-December, an excess of births ranging from 2.8 cases/10,000 births to 7.1 cases/10,000 births was observed. For these three countries, solar radiation levels around conception and the postnatal period were inversely correlated with seasonal trends in ASD risk.

Conclusions: Assuming that season of birth is a proxy for temporally fluctuating environmental conditions, this study provides further support of the involvement of non-genetic risk factors in the etiology of ASD.