Baby boys are more likely to have changes in their genitals — such as undescended testicles and smaller penises — if their mothers were exposed to high levels of a controversial chemical during pregnancy, a new study shows.

Virtually everyone has been exposed to the chemicals, called phthalates, which are used in countless plastic products and are found in everything from drinking water to breast milk to household dust, according to the study, published in the current issue of Environmental Research.

Until recently, most studies have been conducted in animals. Those tests suggest that phthalates interfere with the male sex hormone testosterone, causing a "phthalate syndrome" in male fetuses that changes the way their genitals develop, says study author Shanna Swan, a professor at the University of Rochester School of Medicine and Dentistry.

Swan says her study of 106 mothers and sons suggests this syndrome may be occurring in humans, too.

In her study, doctors measured phthalate levels in the mothers' urine during pregnancy, then examined the babies at 12 months.

Boys whose mothers had the highest phthalate levels were more likely than others to show three anatomic differences: smaller penises, a shorter distance between the anus and base of the penis, and undescended or incompletely descended testicles, Swan says.

Swan also notes that most boys had normal sex organs. Twelve had incompletely descended testicles, while 29 babies fell into a category with "shorter" anogenital distances.

In most cases, these aren't serious problems, Swan says. Babies with undescended testicles often need no treatment, because the organs descend on their own by age 1. Others can be helped with hormone treatments or surgery. And even the smaller penises appeared to be within the normal range.

But Swan says she's concerned that these changes indicate a deeper problem — that phthalates may have made the boys "less masculine" in key ways. In animals, males with these genital changes also had lower sperm counts, she says.

Swan says she is also concerned about girls. It's possible that any effects from pre-birth phthalate exposure may not surface until the girls hit puberty or try to have children, she says.

In her paper, she notes that other researchers have linked phthalates to reduced sperm quality and DNA damage, as well as hormone changes, reduced lung function and premature puberty.

More and more Americans are becoming concerned about phthalates.

In August, Congress banned several types of the chemicals in children's toys and products. Dozens of hospitals around the country are phasing phthalates out of their neonatal intensive care units to protect vulnerable newborns, who may spend weeks or months connected to plastic tubing.

CONGRESS BANS:Congress moves to ban contoversial chemical

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Swan says scientists need to know more about the cumulative effect of these chemicals, since people are exposed to a variety of phthalates and other hormone-like chemicals every day. She also says products made with phthalates should be clearly labeled.

"People should have a choice about limiting their exposure," Swan says.

Swan notes that the design of her study has limitations. Although the study links phthalates with these anatomical changes, it doesn't prove cause and effect. In her study, she notes that larger, stronger studies cost at least $1 million and take five years or more.

The American Chemistry Council, which represents the chemical industry, says on its website that phthalates are safe. "The trace levels" of phthalates found in humans are below the Environmental Protection Agency's safety guideline, so they shouldn't threaten human health, the council says.

In a statement on its website, the council says: "Phthalates have established a very strong safety profile over the 50 years in which they have been in general use. There is no reliable evidence that any phthalate has ever caused a health problem for a human from its intended use."