Pregnant women with high levels of the pesticide DDT have a higher risk of giving birth to a child with autism, a new study reveals.

The controversial chemical DDT made its mark as a powerful pesticide in the early 1900s, but is now banned in most countries around the world for its effects on the environment, wildlife, and human health.

DDT And Its Effects On Autism

The study, published in the American Journal of Psychiatry, examined blood samples from over a million mothers in Finland who were pregnant from 1987 to 2005. From this pool, the team identified 778 children diagnosed with autism, then pulled 778 children without the diagnosis to serve as control subjects.

After analyzing the blood samples of the children's mothers, the researchers found that pregnant women with higher concentrations of DDE — the metabolic by-product of DDT — are 32 percent more likely to give birth to children with autism than those with lower levels of DDE.

Mothers with elevated DDE levels were also shown twice as likely to have children with both autism and an intellectual disability.

According to Live Science, the results stayed consistent even when the researchers adjusted other factors such as the mother's age, socioeconomic status, and a history of psychiatric disorders.

The study provides the first biomarker-based evidence that a mother's exposure to insecticides is associated with autism.

The researchers also tested the blood samples for another banned chemical PCB, but no links were found to autism.

Potential Reasons Behind The Association

There is still a lack of clear reason behind DDT's significant impact on autism. However, the study's lead researcher Alan Brown, a psychiatrist and epidemiologist at Columbia University, offers two potential explanations, according to Nature. One is DDT's known effect of low birth weight and premature birth, both of which are related to autism.

The chemical also binds to proteins known as androgen receptors, which has been shown in rodent studies to interfere with fetal brain development.

Still, Brown says the overall risk of autism remains low, even in children whose mothers were found with higher concentrations of DDE.

While the findings were significant, the authors also stress that the study does not prove that DDT directly causes autism. It may simply be a single contributor in a myriad of factors that lead to the disorder.

"Very likely, you need other predisposing factors [for autism] in addition to [DDE]," Brown explains to Live Science. "I don't think moms should be going out and getting tested for these things."

DDT In The Environment

DDT has been banned for a few decades in many countries, but it's still occasionally used in Africa to keep mosquito populations down.

It is also a chemical that breaks down extremely slowly, so DDT is still lingering in the soil and water today.