The paper, published today in the journal Environmental Health Perspectives, found that the more a mother was exposed to the chemicals, called phthalates (THAL-ates), the greater the chance her boy's reproductive development would be harmed. Similar changes have led to decreased semen quality and fertility in rodents.

"We'll follow our children to see what the consequences are," says lead researcher Shanna Swan, a professor of epidemiology at the University of Rochester (N.Y.) School of Medicine.

The changes described in the federally financed study were seen at phthalate levels found in one-quarter of the female population in the USA.

Effects in Humans

The study tested levels of four kinds of phthalates in the urine of pregnant women. Researchers later examined 134 of the baby boys 2 months to 30 months old who were born to those women.

Previous work had shown that prenatal phthalate exposure in rodents can critically affect male hormones, resulting in impaired testicular descent and smaller genital size. The Swan study is the first to look at effects in humans.

While none of the boys showed clear malformation or disease, in the 25% of mothers with the highest levels of phthalate exposure, the odds were 10 times higher that their sons would have a shorter than expected distance between the anus and the base of the penis.

This so-called AGD measurement is a sensitive indicator of impacts on their reproductive system.

'Public Health Implications Are Enormous'

A statement from the Cosmetic Toiletry and Fragrance Association said the "use of phthalates in cosmetics and personal care products is supported by an extensive body of scientific research and data that confirms safety."

But, Swan says, no one had ever studied phthalate exposure in infant boys.

"It's way premature to have concern," says Marian Stanley, who manages the Phthalate Esters Panel of the American Chemistry Council in Arlington, Va. "More study is needed."

Andrea Dunaif, chief of endocrinology at Northwestern University, called the findings "strong evidence in humans that this endocrine-disrupting chemical is associated with changes in boys."

The changes are subtle, but male infertility rates appear to be rising, she said, and it's hard to know if the problem is environmental or just diagnosed more often. "The public health implications are enormous."