FRESNO, Calif. – Pregnant women who live near agricultural fields where pesticides are sprayed are at increased risk of having a child with autism, a study suggests.

The study by the University of California, Davis, MIND Institute found mothers exposed to organophosphates had a two-thirds increased risk of having a child with autism. And the risk was strongest when exposures occurred during the second and third trimesters of pregnancies, the research showed.

UC Davis researchers said their results strengthen the evidence of an association found in a study seven years ago based in California's Central Valley. Families living close to agricultural fields may want "to leave town or keep their children away or close the windows" on days when pesticides are being applied, said Dr. Irva Hertz-Picciotto, senior author of the report and professor and vice chair of the Department of Public Health Services at UC Davis.

UC Davis said the study, published online in Environmental Health Perspective, used commercial pesticide application data from the California Pesticide Use Report, which shows what pesticides are applied, and where and when they are applied.

Using the pesticide data maps, researchers overlaid addresses of about 1,000 participants in a Childhood Autism Risks from Genetics and Environment study — a population-based, case-control study of children ages 2-5 with autism, developmental delay and typical development.

The risk of an autism spectrum disorder decreased the farther the pregnant women lived from where pesticides were sprayed, the report said.

The research included four classes of pesticides: organophosphates, organochlorines, pyrethroids and carbamates. According to the study, the spraying of organophosphates while a woman was pregnant was linked to an elevated risk of autism, particularly the application of the pesticide chlorpyrifos in the second trimester and organophosphates in the third trimester.

Pyrethroids also were associated with autism and developmental delay prior to conception or in the third trimester. Carbamates sprayed nearby while a woman was pregnant were associated with developmental delay, the study said.

There is no known cause for autism, but environmental factors have been one focus of study for the past several years.

Although more research needs to be done to learn how pesticides might influence brain development, it's clear that most are neurotoxic, the researchers said. "A number of pesticides impact the balance of neuro-excitation and neuro-inhibition" that control mood, learning, social interactions and behavior, UC Davis' Hertz-Picciotto said.