Exposure to several common agricultural pesticides during pregnancy increases the risk of developmental delays and autism in children by two-thirds, a new study found. While researchers did not say pesticides cause autism, a direct link is plausible.

Researchers at the University of California, Davis’ MIND Institute tracked associations with specific classes of pesticides (including organophosphates, pyrethroids and carbamates) and later diagnoses of autism and developmental delay in children. They used maps from the California Pesticide Use Report (1997-2008) and the addresses of expectant mothers to track women’s exposure to agricultural pesticide spraying during their pregnancies.

Developmental delay, in which children take extra time to reach communication, social or motor skills milestones, affects about four percent of US. kids, the authors wrote. The Centers for Disease Control and Prevention estimates that one in 68 children has an autism spectrum disorder (ASD), also marked by deficits in social interaction and language.

Of the 970 children covered by the study, 486 had an ASD, 168 had developmental delays and 316 had typical development.

"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," principal investigator Irva Hertz-Picciotto, a MIND Institute researcher and professor and vice chair of the Department of Public Health Sciences at UC Davis, said in a statement. "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."

The Northern California-based Childhood Risk of Autism from Genetics and the Environment (CHARGE) Study was published online in Environmental Health Perspectives. It found that approximately one-third of the study participants lived in “close proximity” (just under one mile) of commercial pesticide application sites.

“This study of ASD strengthens the evidence linking neurodevelopmental disorders with gestational pesticide exposures, and particularly, organophosphates and provides novel results of ASD and DD associations with, respectively, pyrethroids and carbamates,” the researchers said in the study.

Proximity to organophosphates at some point during gestation was associated with a 60 percent increased risk for ASD, researchers said. Many insecticides are organophosphates, which kill insects by disrupting their brains and nervous systems.

"Many of these compounds work on neurons. When they work on the insect, they're dealing with the nervous system of the insect and basically incapacitating it," Hertz-Picciotto said to HealthDay Reporter.

Autism risk was also increased with exposure to so-called pyrethroid insecticides, as was the risk for developmental delay. Pyrethroids are often sprayed to kill mosquitoes to prevent the spread of West Nile virus, and are similar to the natural pesticide pyrethrum, which is produced by chrysanthemum flowers. Because of the link to mums, this class of insecticides can sometimes be labeled as “all natural,” Hertz-Picciotto told HealthDay.

"It's a synthetic product that's been designed to be more toxic than the natural product it's imitating," she said.

Carbamate pesticides were linked to developmental delay but not ASDs, the study found. Carbamates are commonly used as surface sprays or baits in the control of household pests.

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

“We already knew from animal studies as well as from epidemiologic studies of women and children that prenatal exposure (to pesticides) is associated with lower IQ,” Landrigan told Reuters Health. “This study builds on that, uses the population of a whole state, looks at multiple different pesticides and finds a pattern of wide association between pesticide exposure and developmental disability.”

“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," lead author Janie F. Shelton, from the University of California, Davis, said in the statement.

In an email to Reuters Health, Shelton said scientists need to do more research before they can say that pesticides cause autism, but, because pesticides all affect signaling between cells in the nervous system, a direct link is plausible. “Ours is the third study to specifically link autism spectrum disorders to pesticide exposure, whereas more papers have demonstrated links with developmental delay,” she said.

Hertz-Picciotto and Landrigan recommend limiting exposure to neurotoxins by not using insecticides containing organophosphates or pyrethroids in the months before and during pregnancy.

"I think it’s an area that people do need to think about, both at the individual level…if they can make some choices, it may be worth it to them," Hertz-Picciotto said to Newsweek. "I don’t use chemical pesticides that are toxic. I know it takes sometimes a little longer. I’m willing to live with those extra couple days when there might be creepy crawly things."