Pregnant women who lived in close proximity to fields and farms where chemical pesticides were applied experienced a much higher risk of having a child with autism spectrum disorder or other developmental delay, according to the findings of a new study by UC Davis California researchers. The associations were stronger when the exposures occurred during the second and third trimesters of the women's pregnancies.

The University of California study looked at the associations between different classes of pesticides, including organophosphates, pyrethroids and carbamates, applied during the study participants' pregnancies and later diagnoses of autism and other developmental issues in children, using participants from across the State.

"This study validates the results of earlier research that has reported associations between having a child with autism and prenatal exposure to agricultural chemicals in California," said lead study author Janie F. Shelton, "While we still must investigate whether certain sub-groups are more vulnerable to exposures to these compounds than others, the message is very clear: Women who are pregnant should take special care to avoid contact with agricultural chemicals whenever possible."

The study showed that living in close proximity to commercial pesticide application areas was associated with a risk two-thirds higher than living further away from applications.

California is the top agricultural producing state in the USA, with farm crops grossing around $38 billion (£22bn) each year. Approximately 90,000 tonnes of active pesticides are applied each year across the State. There is mounting evidence to suggest certain commonly used pesticides are neurotoxic to humans as well as the insect pests they are intended to target, and may pose threats to brain development during pregnancy.

The UC Davis study was conducted by examining commercial pesticide application using the California Pesticide Use Report and linking the data to the residential addresses of approximately 1,000 participants in another California-based Study.

Twenty-one chemical compounds were identified in the organophosphate class, including chlorpyrifos, acephate and diazinon. The second most commonly applied class of pesticides was pyrethroids, one quarter of which was esfenvalerate, followed by lambda-cyhalothrin permethrin, cypermethrin and tau-fluvalinate. Eighty percent of the carbamates were methomyl and carbaryl.

Commenting on the findings, Irva Hertz-Picciotto, who contributed, said, "We mapped where our study participants lived during pregnancy and around the time of birth. In California, pesticide applicators must report what they're applying, where they're applying it, dates when the applications were made and how much was applied. What we saw were several classes of pesticides more commonly applied near residences of mothers whose children developed autism or had delayed cognitive or other skills."

Organophosphates applied over the course of pregnancy were associated with an elevated risk of autism spectrum disorder, particularly for chlorpyrifos applications in the second trimester. Pyrethroids were moderately associated with autism spectrum disorder immediately prior to conception and in the third trimester. Carbamates applied during pregnancy were associated with developmental delay.

Exposures to insecticides for those living near agricultural areas may be problematic, the researchers said, especially during gestation, because the developing foetal brain may be more vulnerable than it is in adults. Because these pesticides are neurotoxic, in utero exposures during early development may distort the complex processes of structural development and neuronal signaling, possibly altering mechanisms that govern mood, learning, social interactions and behavior.

"In that early developmental gestational period, the brain is developing synapses, the spaces between neurons, where electrical impulses are turned into neurotransmitting chemicals that leap from one neuron to another to pass messages along. The formation of these junctions is really important and may well be where these pesticides are operating and affecting neurotransmission," Hertz-Picciotto added.

Hertz-Picciotto said that, in light of the study's findings, seeking ways to reduce exposures to chemical pesticides, particularly for the very young, is important. "We need to open up a dialogue about how this can be done, at both a societal and individual level," she said. "If it were my family, I wouldn't want to live close to where heavy pesticides are being applied."