Objectives: The objective of this study was to address many of the limitations of previous studies by using a large, national, population-based sample to investigate the association between seasonality and all subtypes of ASD. Birth month of individuals diagnosed with each ASD diagnostic subtype (Infantile Autism, Asperger’s Syndrome and Pervasive Developmental Disorder), rather than “season”, which may be defined differently in various regions, was examined for deviation from the monthly pattern of birth of children in the general population without autism.
Methods: All ASD cases born in 1990-2002 were identified from the Finnish Hospital Discharge Register (FHDR). Information concerning total births in Finland in 1990-2002 was obtained from the Finnish Medical Birth Registry (FMBR). Descriptive Chi-Squared “goodness-of-fit” tests, adjusted for year of birth, were conducted to examine how well observed frequencies of births of children with ASD differed from expected frequencies. Females and males were analyzed separately in order to examine any differences by gender. Poisson regression analyses will be used to test the data against a sinusoidal function.
Results: Preliminary results examining all births revealed a deviation in birth pattern in the months of June (p=.01) and October (p=.06) for all three subtypes of ASD. There was an association between a decrease in births of females diagnosed with all ASD subtypes (p=.04) in the month of June. There was an increase in males diagnosed with infantile autism born in October (p=.05), and the month of December approached significance (p=.08).
Conclusions: These results suggest that environmental exposures may contribute to increased risk of ASD in males. The finding that females born in June are at decreased risk of being diagnosed with ASD warrants further exploration.