Other contributors to the increasing incidence of autism remain unclear. "We're doing a lot of research into environmental risk factors," Shelton says, describing ongoing research into possible nutritional factors and toxic chemical exposure during labor and development. It is possible that the increased risk associated with maternal age might reflect the mother's longer cumulative exposure to unknown environmental factors, the authors report.
The research team published an earlier report in the same journal describing high-incidence geographic clusters in California, another finding in line with Bearman's work that suggests environmental processes and social influences (why someone would live in a particular neighborhood) might be contributing factors. Maternal autoimmunity is another theory proposed by the researchers, who previously reported that some mothers of autistic children had antibodies to fetal brain proteins in their plasma. These antibodies (which might increase in number with age) could transfer into the fetus and interfere with early brain development, the researchers report.
Whereas biomedical studies are required to uncover the mechanisms underlying the disorder, Shelton says the present epidemiological study was important in clarifying the nuanced relationship between maternal age and autism, and defining its contribution to the rise in cases. It might have even provided biological clues. "It really is a maternally mediated biological process that's going on," Shelton says.
Although it is rising, the risk of autism is still very low and shouldn't affect the decision to have children at any age, Shelton says. "People should pursue their families whenever it's right for them," she says, adding that soon-to-be parents should "just stay as healthy as possible," and steer clear of dangerous exposures. She also encourages parents with autistic children to get involved in research. "I think parents are anxious because science hasn't figured it out yet. If they have the opportunity to be involved in supporting science and autism research, that's a great thing."