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A new study from California has found that children with an autism spectrum disorder were more likely to have mothers who lived close to fields treated with certain pesticides during pregnancy.

Proximity to agricultural pesticides in pregnancy was also linked to other types of developmental delay among children.

"Ours is the third study to specifically link autism spectrum disorders to pesticide exposure, whereas more papers have demonstrated links with developmental delay," said lead author Janie F. Shelton, from the University of California, Davis.

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There needs to be more research before scientists can say that pesticides cause autism, she said. But pesticides all affect signalling between cells in the nervous system, she added, so a direct link is plausible.

California is one of only a few states in the U.S. where agricultural pesticide use is rigorously reported and mapped. For the new study, the researchers used those maps to track exposures during pregnancy for the mothers of 970 children.

The children included 486 with an autism spectrum disorder (ASD), 168 with a developmental delay and 316 with typical development. Developmental delay, in which children take extra time to reach communication, social or motor skills milestones, affects about 4 per cent of U.S. kids, the authors write. The Centers for Disease Control and Prevention estimates that one in 68 children has an ASD, also marked by deficits in social interaction and language.

In the new study, about a third of mothers had lived within a mile of fields treated with pesticides, most commonly organophosphates. Children of mothers exposed to organophosphates were 60 per cent more likely to have an ASD than children of non-exposed mothers, the authors report in Environmental Health Perspectives.

Autism risk was also increased with exposure to pyrethroid insecticides, as was the risk for developmental delay. Carbamate pesticides were linked to developmental delay but not ASDs.

For some pesticides, exposure seemed to be most important just before conception and in the third trimester, but for others it didn't seem to matter when during pregnancy women were exposed.

Dr. Philip J. Landrigan speculated that pesticides drifted from crops through the air. The new study did not measure airborne pesticide levels, however. Landrigan directs the Children's Environmental Health Center at the Icahn School of Medicine at Mount Sinai in New York and was not involved in the new study.

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"One lesson or message for parents is to minimize or eliminate use of pesticides in their own homes," Landrigan said.

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