The prevalence of autism spectrum disorders (ASDs) is causally dependent on genetic and environmental influences. We investigated whether autism spectrum disorders are associated with month of birth compared to the general population using a retrospective study, comparing ASD cases (n = 3478) with the general population (n = 2,716,876) born between 1995 and 2008. Associations were examined using χ2 tests and Walter and Elwood’s seasonality χ2 tests for the total ASD group, and separately for autistic disorder and Asperger syndrome. For the total ASD group, the distribution of month of birth was different compared to the general population (p < 0.0001), with July as the highest contributor, and a season-of-birth effect was found for this group (p = 0.02). For the autistic disorder group, the months of birth distribution were different (p = 0.01), with July as the highest contributor. No season-of-birth effect over the year was found (p = 0.09). No association was found for the months of birth of individuals with Asperger syndrome (p = 0.06), with no seasonal trend over the year (p = 0.60). In conclusion, a drop in sun exposure during the first trimester of pregnancy might explain the peak in July births and the associated risk for ASD development.