Earlier this week, the LA Times ran a provocative article under the questioning headline above. It suggested that autism’s twentyfold increase over the last generation may be “more of a surge in diagnosis than in disease.” In fact, scientific evidence suggests that autism’s dramatic increase is only partially explained by improved screening and diagnosis.
Some of the clearest evidence of this increase comes from research documenting a 600 percent jump in autism caseload in California between 1992 and 2006. In related studies (here and here), Peter Bearman estimated that around 42 percent of the increase can be explained by changes in diagnostic methods and awareness with another 11 percent possibly due to increases in parental age at the time of conception (a known risk factor).
Taking into account all the factors that have been studied, this leaves approximately half of the increase due to still-unidentified factors. Through research, we’re increasing our understanding of these influences. For example, we now know that prematurity and extreme low-birth weight increase autism risk in babies. Certainly survival rates for premature and very low birth weight infants have increased considerably over the last twenty years.
While no single factor is likely to explain the marked increase in autism’s prevalence, researchers agree that a number of influences likely work together to determine the risk that a child will develop an autism spectrum disorder (ASD).
Bottom line: It is undeniable that more children are being diagnosed with ASD than ever before. The need for increased funding for autism science and services has never been greater. Autism costs society is a staggering $35 billion per year. And with more cases, that figure is likely to increase. Fortunately, there is clear evidence that earlier identification and intervention and supports throughout the lifespan can improve outcomes and quality of life.
If you are concerned about your child’s development, please see the “Learn the Signs” page of our website. If you are an adult struggling with issues that might be related to autism, please follow the hyperlinks to our resource page for adults and our page on Asperger Syndrome.
Got more questions? Send them to GotQuestions@autismspeaks.org. And join our next live webchat with Dr. Dawson and her co-host, Autism Speaks assistant vice president and head of medical research Joe Horrigan, MD on January 5th. More information on their monthly webchats here.
Is there empirical evidence that parental age is a contributing factor to giving birth to a child with autism?
“Got Questions?” is a new weekly feature on our blog to address the desire for scientific understanding in our community. We received over 3000 responses when we asked what science questions were on your mind. We answered a few here and the Autism Speaks Science staff will address the other themes we received in this weekly post.
A number of recent publications investigating the relationship between parental age and autism have demonstrated that older parents are at increased risk for having a child with autism. This is not surprising since increased parental age is associated with a slightly increased risk for other developmental disorders, as well. Grether, et. al. reported in 2009 that parental age and particularly maternal age is a significant risk factor for autism. The authors found that a 10-year increase in maternal age increased the chances of having a child with autism by 38% and mothers over the age of 40 were at highest risk. Other studies, such as a recent study by Hertz-Piccioto, found that the risk for autism spectrum disorder (ASD) in older mothers was lower than 38%. King and Bearman, et. al. similarly found that older mothers and fathers were at increased risk of having a child with autism with the largest risk among mothers aged 40 and over. Since the risk for ASD is low, the risk for ASD in older mothers, although increased, is still relatively low. The underlying mechanism behind the relationship between increased parental age and risk for autism is currently unknown and under investigation.
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.