For a variety of studies I asked the same question: How large is the increased risk for autism? My standard for comparison was the likelihood in the general population of autism spectrum disorder. Here’s an example. Start from the fact that the recorded rate of autism is now 1 in 68, according to a report released last week by the Centers for Disease Control and Prevention. If babies born in purple farmhouses have a rate of autism of 2 in 68, this doubling means that the purple farmhouse carries a risk ratio of 2. However, correlation is not causation, and there is no need to repaint that farmhouse just yet.
We can improve our chances of finding true causes by looking before the age of 2, when it becomes possible to diagnose autism. The risk ratio can give perspective where isolated news stories don’t. Media reports have focused on the risk associated with becoming a mother or father in one’s late 30s or after. The story has obvious appeal: Delayed parenthood is common, and readers are understandably anxious. However, parents-to-be should consider that the individual risk to the child is only around 1.4. The risk associated with enhanced or accelerated labor in full-term babies is about 1.2, after other complications are taken into account. And of course, the risk from vaccination is slightly less than 1 — there is no added risk. Even worse, incorrect beliefs about vaccines come with a cost. The return of measles in communities with falling vaccination rates is one recent example.
A highly underappreciated prenatal risk is stress. For pregnant women who take the sometimes-wrenching step of emigrating to a new country, for example, the risk ratio is 2.3. In the fifth through ninth months of pregnancy, getting caught in a hurricane strike zone carries a risk ratio of about 3. Maternal post-traumatic stress disorder during pregnancy is associated with a similar effect. These events are likely to trigger the secretion of stress hormones, which can enter the fetus’s bloodstream and affect the developing brain for a lifetime. Stressors may also lead to maternal illness, the immune response to which may interfere with brain development.