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Boys exposed in the womb to high levels of a chemical found in vinyl products are born with slightly altered genital development, according to research published today.

The study of nearly 200 Swedish babies is the first to link the chemical di-isononyl phthalate (DiNP) to changes in the development of the human male reproductive tract.

Previous studies of baby boys in three countries found that a similar plastics chemical, DEHP, was associated with the same type of changes in their genitalia.

Less is known about the reproductive risks of DiNP, a chemical which scientists say may be replacing DEHP in many products such as vinyl toys, flooring and packaging. In mice, high levels block testosterone and alter testicular development.

“Our data suggest that this substitute phthalate may not be safer than the chemical it is replacing,” wrote the researchers, led by Carl-Gustaf Bornehag at Sweden’s Karlstad University, in the journal Environmental Health Perspectives.

Levels of DiNP in U.S. adults and children have more than doubled in the past decade.

“This study raises concern about DiNP, which is being used in increased amounts in products that contain vinyl plastics, and the impact on the developing fetus,” said Dr. Russ Hauser, a professor of environmental and occupational epidemiology at Harvard School of Public Health who is not involved in the new study.

The researchers measured metabolites of five phthalates in the urine of pregnant women during the first trimester. Development of male reproductive organs begins during that period, said senior study author Shanna Swan, a professor of reproductive science at Mount Sinai Hospital in New York.

The researchers then measured the anogenital distance – the length between the anus and the genitals – when the boys were on average 21 months old. Boys who had been exposed to the highest levels of DiNP in the womb averaged a distance that was slightly shorter – about seven-hundredths of an inch – than the boys with the lowest exposures.