Variation by diagnostic subtype in risk for autism spectrum disorders associated with maternal parity among Finnish...

Associations between maternal parity and outcomes in offspring may provide evidence for involvement of prenatal exposures. The objective of this study was to determine whether risk for autism spectrum disorders (ASD) is associated with maternal parity.

Diagnoses of childhood autism, Asperger syndrome, and pervasive developmental disorder, not otherwise specified (PDD-NOS) were examined separately and as a group. The study was conducted in the Finnish Prenatal Study of Autism, which is based in a national birth cohort. Children born in Finland in 1987-2005 and diagnosed with ASD by 2007 were identified through the Finnish Hospital Discharge Register. Four matched controls were selected for each case using the Finnish Medical Birth Register. The association between parity and each ASD was determined using conditional logistic regression and adjusted for number of children in the sibship and other potential confounders.

ASDs combined showed a pattern of decreasing risk with increasing parity (odds ratio OR for fourth or greater vs. first-born children, 0.43 [95% confidence interval (CI): 0.35, 0.51]). For childhood autism, an adjusted OR of 1.51 [95% CI 1.27, 1.81] was observed for second vs. first-born children. Associations for Asperger syndrome and PDD-NOS were consistent with those for all ASDs.

Differences in patterns of association between maternal parity and ASD subtypes may indicate varying contributions of specific environmental factors to risk; however, differences in diagnosis or in treatment seeking for childhood behavioural problems cannot be ruled out, particularly for higher functioning cases.