Autism spectrum disorders (ASD) have been on the rise. Children with ASD have trouble relating to others social and may have difficulty communicating. They may also show repetitive behaviors and interests, and cognitive delays.

No specific cause for ASD has been found, but researchers have identified many risk factors including the growing presence of environmental pollutants. And now a new large-scale study provides compelling evidence linking autism with a mother's exposure to agricultural pesticides.

Pregnant women who lived near areas treated with pesticides had a greatly increased risk of having a child with ASD or developmental delays.

Researchers in California found that pregnant women who lived near areas treated with pesticides had a greatly increased risk of having a child with ASD or developmental delays. This risk was even higher when the exposures occurred during the second and third trimesters.

“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…the message is very clear: Women who are pregnant should take special care to avoid contact with agricultural chemicals whenever possible,” Janie Shelton, lead author said in a statement.

As the most agriculturally productive state in the US, California uses approximately 200 million pounds of active pesticides each year. While the modern agriculture industry may depend on these measures, certain commonly used pesticides are toxic to the nervous system. They can also affect fetal brain development during pregnancy.

The research focused on risks associated with specific types of pesticides including organophosphates, pyrethroids and carbamates. “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,” said Irva Hertz-Picciotto, lead investigator and a professor at UC Davis.

“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,” she added.

Organophosphates were associated with an increased risk of ASD, particularly for chlorpyrifo exposure during the second trimester. Pyrethroids were moderately associated with ASD immediately prior to conception and in the third trimester. Carbamates, on the other hand, were linked with developmental delays.

Many insecticides have neurotoxic properties and pose a particular risk to the fetus during pregnancy. The developing brain is more vulnerable than an adult brain, and exposure to pesticides may distort the complex process of structural development and signaling. As a result, mood, learning, social interaction and behavior may be permanently altered.

It’s impossible to entirely eliminate risks due to environmental exposures, however we must find ways to reduce exposures to chemical pesticides, particularly for the very young, argues Hertz-Picciotto.

“If it were my family, I wouldn't want to live close to where heavy pesticides are being applied.”

The study is published in Environmental Health Perspectives.