Autism risk associated with parental age and with increasing difference in age between the parents

Advancing paternal and maternal age have both been associated with risk for autism spectrum disorders (ASD). However, the shape of the association remains unclear, and results on the joint associations is lacking. This study tests if advancing paternal and maternal ages are independently associated with ASD risk and estimates the functional form of the associations. In a population-based cohort study from five countries (Denmark, Israel, Norway, Sweden and Western Australia) comprising 5 766 794 children born 1985–2004 and followed up to the end of 2004–2009, the relative risk (RR) of ASD was estimated by using logistic regression and splines. Our analyses included 30 902 cases of ASD. Advancing paternal and maternal age were each associated with increased RR of ASD after adjusting for confounding and the other parent's age (mothers 40–49 years vs 20–29 years, RR=1.15 (95% confidence interval (CI): 1.06–1.24), P-value<0.001; fathersgreater than or equal to50 years vs 20–29 years, RR=1.66 (95% CI: 1.49–1.85), P-value<0.001). Younger maternal age was also associated with increased risk for ASD (mothers <20 years vs 20–29 years, RR=1.18 (95% CI: 1.08–1.29), P-value