Nov. 16, 2009 -- Mothers exposed to high levels of chemicals known as phthalates during pregnancy may have boys who are less likely to play with trucks and other male-typical toys or to play fight, according to a new study.

Phthalates, common in the environment, are found in toys, food packaging, personal care products, nail polish, adhesives, and other products.

In the study, the researchers focused on two phthalates of concern to environmental experts, DEHP and DBP. They tested the urine of women during the 28th week of pregnancy and divided them into four groups depending on the concentration of phthalate metabolites or breakdown products. Then they assessed the play behavior of the 145 children when they reached age 3 to 6.

If mothers were in the highest concentration group, the chance that their boys had a less masculine score was five times greater than mothers in the lowest concentration group, according to study researcher Shanna Swan, PhD, professor of obstetrics and gynecology at the University of Rochester Medical Center's Center for Reproductive Epidemiology and an expert on phthalates.

''I'm not saying these boys are feminized," Swan tells WebMD. Rather, she says, ''they are less likely to play in a male-typical manner." No effect was found with the girls.

Swan and other experts suspect that exposure to the chemicals affects the level of testosterone crucial for the development not only of male reproductive organs, but also the masculine brain. ''We now suspect that the phthalate [exposure] affects the entire body, not just the reproductive tract,'' Swan says.

The study is published in the International Journal of Andrology.