Autism Speaks Official Blog

These numbers may sound alarming, but lets further discuss what they mean.  The 51% increase in risk for mothers over 40 can also be described as approximately 1.5 times the chances of having a child with autism compared to mothers 25-29.  In other words, the increased chance of having a child with autism was less than two-fold among this group. Thus, mother’s age and father’s age only slightly increased the risk for autism, and should not be viewed as a specific cause of autism. While the exact biological mechanism behind the relationship between delayed parenthood and ASD is unknown and warrants further investigation, it is well understood that pregnancy in older individuals is associated with higher risk for low birth weight, prematurity, and chromosomal abnormalities.  We also know that prematurity is a risk factor for autism spectrum disorders (ASD).  It also is important to keep in mind that the majority of pregnancies in older fathers and mothers are healthy.

Do changes in the ages at which parents are having children explain the dramatic increase in prevalence of ASD?  The study examined births over a decade, a period during which the prevalence of ASD has increased by over 600%.  The authors estimated that advanced maternal age only accounts for 4.6% of the increase in autism cases in California during the study period.  Thus, it is clear that, while changes in the age at which parents are having children may account for some of the increase in prevalence of autism, a large amount of the increase in prevalence remains unexplained.

As with any study, there are many methodological details to be considered.  The strength of this study lies in the large population considered. These findings reinforce other data reporting parental age is a risk factor for autism.  The study population came from the California Department of Developmental Services (DDS), which is estimated to capture about 75-80% of all “true” autism cases.  To be included in the DDS, parents had to actively seek out services.  As such, parents of autism cases in the DDS are likely to have higher levels of education and socioeconomic status, and are perhaps older than what would be found in a population-based sample. We know from this and other research that higher levels of education and sociecomic status are associated with higher rates autism (perhaps because people in this demographic category are more likely to seek diagnostic services), so it is possible that this study may overestimate the effect of parental age on autism risk. Also, it is worth noting that this study considered cases of autistic disorder only, not diagnoses from the rest of the autism spectrum.

To summarize, it is important to remember that as we dig deeper into different contributions to autism risk, we will uncover different pieces of the larger puzzle that may not seem to fit, at least at first.  Some, pieces, like this one regarding parental age, are especially intriguing because they blend biological with the socio-environmental factor of delayed parenthood.   As for biology, it is also true that as people age, modifications occur in the way the genetic code is read. This field of research known as epigenetics and is one part of the larger study of gene x environment interactions.  The topic of gene-environment interaction has been reaching our community with increasing frequency and so Autism Speaks staff and some Guest Scientists will be offering a series of blog posts specifically on these topics.  Please stay tuned. We look forward to putting this puzzle together with you.

To learn more about the recent findings in autism epidemiology, including additional findings on the effects of parental age, please see our list of the Top Ten Science Accomplishments of 2009.

Reference: Shelton JF, Tancredi DJ and Hertz-Picciotto I (2010) Independent and Dependent Contributions of Advanced Maternal and Paternal Ages to Autism Risk. Autism Research.  3: 1-10.