A new study appearing online today from the American Journal of Psychiatry finds that elevated pesticide levels in pregnant women are associated with an increased risk of autism among their children.

Autism is a complex neurodevelopmental disorder with largely unknown causes. It is characterized by problems with communication, difficulty relating to people and events, and repetitive body movements or behaviors. The study examined whether elevated maternal levels of persistent organic pollutants are associated with autism among children. Persistent organic pollutants are toxic chemicals that adversely affect human health and the environment around the world.

The study examined levels of DDE (p,p’-dichlorodiphenyl dichloroethylene), a breakdown product of the pesticide DDT (dichlorodiphenyltrichloroethane). Although DDT and other persistent organic pollutants were widely banned in many countries decades ago, they persist in the food chain, resulting in continuous exposure among populations. These chemicals transfer across the placenta, resulting in potential prenatal exposure among nearly all children because of existing maternal body burdens.

The researchers evaluated levels of DDE in maternal serum samples drawn from more than 750 children with autism and matched control subjects from a national birth cohort study, the Finnish Prenatal Study of Autism. The odds of autism among children were significantly increased in mothers whose DDE levels were elevated (defined as the 75th percentile or greater). In addition, the odds of children having autism with intellectual disability were increased more than twofold with maternal DDE levels above this threshold. While these results indicate an association, they do not prove causation, although the findings persisted after controlling for confounding factors.

The study also evaluated mothers’ levels of PCBs (polychlorinated biphenyls), chemicals used in industry, and found no association with autism in children.

The authors conclude that their findings “provide the first biomarker-based evidence that maternal exposure to insecticides is associated with autism among offspring.” Although further research is needed, this study contributes to the understanding of autism and has implications for preventing this disorder, the authors note.