Baby boys who are exposed in the womb to a chemical used in soft plastics may show small signs of altered genital development, according to new research published today. 

The study, which included more than 700 infants in four U.S. cities, is the largest of its kind to date. It confirms earlier findings in humans and animals that exposure to certain types of chemicals called phthalates may lead to changes in the way the male reproductive tract develops, said Dr. Russ Hauser, an epidemiologist at the Harvard School of Public Health, who was not involved in the new study.

Phthalates are a large group of industrial chemicals used in a variety of consumer products, such as food packaging, flooring, perfumes and lotions.

The changes seen in the babies in the study were small, said lead author Shanna Swan, a reproductive health scientist at Mount Sinai Hospital in New York.

"There was nothing clinically abnormal or noticeably different about these boys," Swan told Live Science. [12 Worst Hormone-Disrupting Chemicals & Their Health Effects ]

In the study, the researchers measured pregnant women's levels of 11 compounds that are formed in the body when phthalates are broken down. The researchers looked at the levels of these compounds in the women's urine during the first trimester of pregnancy, which is the period when the fetal reproductive tract begins to develop.

They found that newborn boys who were exposed in the womb to the highest levels of one phthalate, called diethylhexyl phthalate (DEHP), had an "anogenital distance" that was 4 percent shorter than that of the boys born to women with the lowest levels. Anogenital distance — a measure of the length between the anus and the genitals — is a marker of reproductive health, wrote the study authors.

The distance between the anus and the genitals is typically 50 to 100 percent longer in males than in females, and a shortened anogenital distance may signal incomplete masculinization, the researchers said.

It's unclear whether the slight alterations seen in the babies in the study could be permanent, or could result in any reproductive health issues. Animal studies suggest that a shortened anogenital distance at birth may signal reproductive abnormalities later in life, and previous studies in humans have linked shorter anogenital distance with testicular abnormalities and semen problems in men. But the researchers would have to follow up with the boys in the study in adulthood to see whether their reproductive health is affected, Swan said.

The researchers found no association between genital development and levels of several other phthalates in boys. None of phthalates tested were associated with altered genitalia in baby girls. 

In rodents, previous research has shown that some phthalates, including DEHP, block the production of male sex hormones by the testes.

The United States banned the use of DEHP and two other phthalates in children's toys and products in 2008 due to reproductive health concerns. But the chemical may still be used in food-processing and packaging materials, and in medical tubing and supplies, according to the study.

A spokesperson from the American Chemistry Council, which represents chemical manufacturers, said that people could be exposed to phthalates from food processing that involves storage in flexible plastic or rubber bags or containers, but most plastic food packaging and storage items are now made with plastics that don’t contain DEHP.

"Information collected by the Centers for Disease Control and Prevention over the last 10 years indicates that, despite the fact that phthalates are used in many products, exposure from all sources combined is extremely low — much lower than the levels established as safe by scientists at regulatory agencies," a spokesperson for the council wrote in an email to Live Science.

Although DEHP is still found in the bodies of most Americans, people's levels of the chemical have decreased over the past decade, as DEHP has been replaced with other plasticizers, Swan said. The levels of DEHP seen in the pregnant women in this newest study, which were measured from urine samples collected between 2010 and 2012, were about 50 percent lower than the levels in urine from mothers obtained in a previous study, between 2000 and 2002, Swan said.

Smaller studies that Swan conducted in 2005 and 2008 also found that prenatal exposure to DEHP at higher levels was associated with altered male genital development.

"We are finding a significant association between male anogenital distance and phthalates at lower and lower levels, which suggests that there may be no safe level of exposure," she said.

While it's extremely difficult to avoid all exposure to plasticizer chemicals, Swan suggested that opting for unprocessed foods could help people reduce their exposure to DEHP and other phthalates used in the packaging and processing of food.

The findings were published online today (Feb. 19) in the journal Human Reproduction.

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