Previous studies suggest that autism may be more common in the offspring of older parents. There is also a positive association between birth order, parental age, and autism, and a number of reports have shown firstborn children to be at increased risk. The aim of this prospective population-based cohort study was to determine whether advancing maternal and paternal ages were each independent risk factors for the development of autism spectrum disorder (ASD) in a large, population-based cohort of US children. Data were obtained from the Centers for Disease Control and Prevention’s Autism and Developmental Disabilities Monitoring Network, a group of 10 participating centers, which monitored 253,347 live births and had complete information from birth certificates on parental age, birth order, and other variables. The study group was comprised of 1251 children aged 8 years from this birth cohort who met the criteria for an ASD set forth in the Diagnostic and Statistical Manual of Mental Disorders, fourth edition, text revision. Following multivariable analysis to control for the other parent’s age, birth order, and other covariables, maternal and paternal ages were each independently associated with autism: the adjusted odds ratio (aOR) for maternal age ≥35 versus 25 to 29 years was 1.3, with a 95% confidence interval (CI) of 1.1 to 1.6; the aOR for paternal age ≥40 years versus 25 to 29 years was 1.4, with a 95% CI of 1.1 to 1.8. There was a 3-fold greater likelihood of autism among firstborn offspring of 2 older parents compared to third- or later-born offspring of mothers aged 20 to 34 years and fathers aged <40 years (aOR, 3.1; 95% CI, 2.0–4.7). The investigators believe that this study is the most compelling evidence to date that the risk of ASD is associated with increases of both maternal and paternal age and decreases with birth order.
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Durkin et al. (2009) studied this question.