Children whose mothers were exposed to low doses of certain pesticides may have a slightly lower IQ by the time they they're in school, three new studies suggest.

The research found children had slightly lower IQs by age 7 if their mothers had higher-than-average exposure in pregnancy to organophosphates.

The organaphosphates studied are insecticides that were once commonly used in residential pest control but now are more likely to be used by farmers on fruits and vegetables.

All three studies, published Thursday in Environmental Health Perspectives, were financed by the National Institute of Environmental Health Sciences and the federal Environmental Protection Agency.

Two were conducted by researchers at Mount Sinai School of Medicine and Columbia University and studied urban families in New York; the third was done by researchers at the School of Public Health at the University of California, Berkeley, and focused on children in the agricultural area of Salinas, Calif.

Each study began a decade ago, when researchers recruited about 1,000 pregnant women. They took blood and urine samples from the women when they were pregnant to measure pesticide exposure and in some cases took umbilical cord blood samples of their children.

The researchers then continued to monitor the health of the children.

In one of the New York studies, the researchers said they found evidence of a link between prenatal exposure to the insecticide chlorpyrifos and deficits in IQ and working memory of the children.

Chlorpyrifos was one of the most widely used insecticides for residential pest control until it was banned for indoor residential use by the U.S. EPA in 2001. All the children studied were born prior to the ban.

The researchers found that those children who had exposures to chlorpyrifos in the upper 25 per cent of the exposure score, scored 5.3 points lower on a test of working memory, and 2.7 points lower on full-scale IQ, compared to those children in the lowest quartile.

"These observed deficits in cognitive functioning at seven years of age could have implications for school performance," lead author Virginia Rauh, deputy director of the Columbia Center for Children's Environmental Health, said in a statement.

"Working memory problems may interfere with reading comprehension, learning and academic achievement, even if general intelligence remains in the normal range."

In the study conducted by researchers from Mount Sinai, women with the highest levels of organophosphates in their urine had children with a slightly lower IQ — for every tenfold increase of a pesticide marker in the mother's urine, children had a 3-point IQ drop at age 7.

In the Berkeley study, children with the highest levels of prenatal pesticide exposure scored seven points lower on intelligence tests compared with children with the lowest levels of exposure. Every 10-fold increase in organophosphate exposure corresponded to a 5.5 point drop in overall IQ scores.

The exposure levels in all the studies were low – in some cases, the levels almost undetectable.

But the study authors note that since the decline in test scores began even at the lowest exposures and continued downward with increasing exposure levels, it may be that no organophosphate exposure level is completely safe.