Environmental Risks

In The Politics of Autism, I discuss various ideas about what causes the condition.

Here is just a partial list of correlates, risk factors, and possible causes that have been the subject of serious studies:

• Pesticides;
• Air pollution and proximity to freeways;
• Maternal thyroid issues;
• Autoimmune disorders;
• Induced labor;
• Preterm birth;
• Birth by cesarean section;
• Maternal and paternal obesity;
• Maternal and paternal age;
• Maternal post-traumatic stress disorder;
• Smoking during pregnancy;
• Antidepressant use during pregnancy. 

At STAT, Alycia Halladay writes:

A mountain of research has been conducted and published on a possible link between vaccines and autism, with hundreds of advocacy and scientific organizations refuting it. They also point out the public health risk of avoiding vaccinations. Yet the fear this unsubstantiated link has generated has led to the re-emergence of vaccine-preventable diseases such as measles and pertussis. Another consequence of the ongoing media attention paid to the issue is a misunderstanding of the real environmental risk factors associated with autism.

... 

Here are a few examples of environmental factors that that have been linked to autism:

• Exposure to the anti-epileptic drug valproic acid during pregnancy
• Older age of the mother or father
• High levels of air pollution during pregnancy. This association has been subjected to a systematic review and withstood scientific rigor. Genes control this association.
• Extreme illness or infection during pregnancy, including severe reactions to bacterial and viral infections (some of which can be prevented through vaccination)
• Obesity during pregnancy, premature birth, and complications during birth, likely as the result of interactions with genes

Exposure to these factors elevates a child’s risk of developing autism anywhere between two and four times. An exhaustive review of these factors was just published in the Annual Review of Public Health.

Large Study of Parental Age as Risk Factor

From Autism Speaks:

The largest-ever multinational study of parental age and autism risk found increased autism rates among the children of teen moms and among children whose parents have relatively large gaps between their ages. The study also confirmed that older parents are at higher risk of having children with autism. The analysis included more than 5.7 million children in five countries.

The study, funded by Autism Speaks, appears online today in the journal Molecular Psychiatry.

...

Specifically the study found:

* Autism rates were 66 percent higher among children born to dads over 50 years of age than among those born to dads in their 20s. Autism rates were 28 percent higher when dads were in their 40s versus 20s.
* Autism rates were 18 percent higher among children born to teen moms than among those born to moms in their 20s.
* Autism rates were 15 percent higher in children born to mothers in their 40s, compared to those born to moms in their 20s
* Autism rates rose still higher when both parents were older, in line with what one would expect if each parent’s age contributed to risk.
* Autism rates also rose with widening gaps between two parents’ ages. These rates were highest when dads were between 35 and 44 years old and their partners were 10 or more years younger. Conversely, rates rose when moms were in their 30s and their partners were 10 or more years younger.

The higher risk associated with fathers over 50 is consistent with the idea that genetic mutations in sperm increase with a man’s age and that these mutations can contribute to the development of autism. By contrast, the risk factors associated with a mother’s age remain unexplained, as do those associated with a wide gap between a mother and father’s age.

The full study is available here.

Maternal Age

New research suggests more emphasis on material age than paternal age as a risk factor. The Wall Street Journal reports on an article in the February issue of Autism Research:

A woman's chance of having a child with autism increases substantially as she ages, but the risk may be less for older dads than previously suggested, a new study analyzing more than five million births found.

"Although fathers' age can contribute risk, the risk is overwhelmed by maternal age,'' said University of California at Davis researcher Janie Shelton, the study's lead author.

Mothers older than 40 were about 50% more likely to have a child with autism than those in their 20s; the risk for fathers older than 40 was 36% higher than for men in their 20s.

A year ago, an HHS publication stressed paternal age.

See here for a related article in the American Journal of Epidemiology