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For every 10-fold increase of the chemicals found in a pregnant women's urine, her pregnancy was reduced by half a week and her child's birth weight decreased about 150 grams. (CBC)

Pregnant women who showed higher exposure to common agricultural pesticides had babies with slightly lower birth weights, a study by Canadian and U.S. researchers has found.

Simon Fraser University health sciences professor Bruce Lanphear and his colleagues tested the urine of 306 pregnant women in Cincinnati, Ohio, twice during their pregnancies. They were looking for chemicals that would show the women had been exposed to organophosphate pesticides, which are used to kill insects.

The telltale chemicals, called metabolites, are generated when the body breaks down organophosphates. Organophosphates represented 36 per cent of all insectides used in the U.S. in 2007, according to the U.S. Environmental Protection Agency.

The study, published in the journal Environmental Health Perspectives this week, found that for every 10-fold increase in the concentration of the chemicals found in a pregnant woman's urine, her pregnancy was reduced by half a week and her child's birth weight decreased about 150 grams.

Lamphear said that's comparable to the decrease in birth weight seen in the babies of women who smoke.

'One of many risk factors'

The amount may seem insignificant for any individual child, he acknowledged.

"But this is just one of many risk factors that a pregnant woman might encounter," he added in a statement. "If a woman has four or five risk factors, the impact can be substantial."

Lower birth weights and premature births are linked to respiratory problems and problems with learning and behaviour.

According to the researchers, the women in the study were exposed to levels of organophosphates comparable to those in most other parts of North America.

Lamphear recommends that pregnant women can reduce their exposure to organophosphate pesticides by choosing organic foods; by washing fruits and vegetables carefully; and by choosing not to use pesticides in and around their homes.