Sunday, October 26, 2008

Autism, Parental Age and Birth Order

A study in the United States has found a link between higher parental age and the likelihood of first born children of older parents developing autism. The study, Advanced Parental Age and the Risk of Autism Spectrum Disorder, by Maureen S. Durkin, Matthew J. Maenner, Craig J. Newschaffer, Li-Ching Lee, Christopher M. Cunniff, Julie L. Daniels, Russell S. Kirby, Lewis Leavitt, Lisa Miller, Walter Zahorodny, and Laura A. Schieve is published in the advance access section of the online edition of the American Journal of Epidemiology, October 21, 2008.

The study defined "older" parents as mothers aged 35 or older and fathers aged 40 or older (maternal age ≥35, paternal age ≥40 years). The risk of developing ASD increased significantly with each 10-year increase in both maternal age and paternal age. Each 10-year increase in maternal age was associated with a 20% increase in ASD risk while each 10-year increase in paternal age was associated with a 30% increase in ASD risk.

The study also confirmed earlier studies showing greater risk of ASD development amongst first born children of older parents:

"The risk of ASD within each of 3 parental age categories (both parents "younger," 1 parent "older," and both parents "older") was highest among firstborn children and declined with increasing birth order" ....... The results of this study provide the most compelling evidence to date that ASD risk increases with both maternal and paternal age and decreases with birth order."

What I find curious is the greater risk of autism development amongst firstborn children given that the parental age will always be greater amongst the subsequently born children than amongst first born. The authors of the study offer no conclusion on this point but provide several existing hypotheses as possible explanations:

The observation in this and at least 2 previous studies (2, 4) that the risk of developing ASD was highest for firstborn children and declined with increasing birth order is a pattern also observed for other childhood disorders, including type I diabetes and atopy, and is cited as support for the "hygiene hypothesis." According to this hypothesis, firstborn children are exposed to fewer infections from other children early in childhood and, because of delayed immunologic challenge, may be more likely to develop autoimmune responses including those that may adversely affect neurodevelopment (29). Another possible factor that could lead to the observed birth-order effect is exposure to potentially neurotoxic, fat-soluble chemicals accumulated in maternal tissue that have been passed to offspring transplacentally or through breast milk (30). Because of accumulation over a lifetime, the load of such neurotoxins transmitted might be expected to be highest for firstborn children, particularly when combined with advanced maternal age. Another possible explanation for the observed birth order effect is "stoppage" or a tendency for parents of 1 child with ASD not to have subsequent children because of the demands of parenting a child with a disability or concerns about genetic susceptibility (31), thus increasing the likelihood in the cohort as a whole that a child with ASD will have a low birth order. Information available for the present study did not allow examination of these hypotheses.

I was 42 when my younger son Conor, subsequently diagnosed with Autistic Disorder, was born. As an older father within the definition of this study I hope that this line of study continues to be explored objectively and professionally and is not derailed or distorted by the anger, from all "sides", that afflicts much public discussion of autism causes and treatments.

Anyone interested in following this issue in the "blogosphere" should check out the blog AUTISM PREVENTION FATHER BABIES 25-33 PATERNAL AGE IS A KEY IN NON-FAMILIAL AUTISMVaccines by concernedheart which is listed on my Autism Blog List in the right side bar of this blog. Concernedheart has been following this area for awhile and updates regularly.

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