Scientists studying the human effects of hormone-mimicking chemicals have reported that compounds called phthalates used in plastics and beauty products, and widely found in people, seem to alter reproductive organs of baby boys.

In the first study of humans exposed in the womb to phthalates, the researchers, who examined the genitalia of male babies and toddlers, found a strong relationship between the chemicals and subtle changes in the size and anatomy of their genitals. Phthalates are ubiquitous compounds used as softeners in plastics and to maintain color and fragrance in beauty products such as nail polish and perfume, among other uses.

It is the first time that scientists have shown that any industrial compound measured in mothers’ bodies seems to disrupt the reproductive systems of their babies.

But many experts, including the authors of the report published today in the online version of the journal Environmental Health Perspectives, say that more research must be done to determine whether the genital abnormalities in the boys lead to fertility or health problems and to prove that they are caused by phthalates.

The findings were based on tests of 85 mothers and sons, averaging nearly 13 months of age, born in Los Angeles, Minneapolis and Columbia, Mo.

Mothers with the highest levels of chemicals in their urine late in pregnancy had babies with a cluster of effects: The span between the anus and penis, called anogenital distance, was comparatively short, and the infants had smaller penises and scrotums and more cases of incomplete descent of testicles.

Medical experts do not know whether babies with those physical characteristics will later develop reproductive problems. But in newborn animals, laboratory studies show that combination of effects can lead to lower sperm counts, infertility, reduced testosterone and testicular abnormalities when they mature.

“In rats it’s called the phthalate syndrome. What we found for the first time is evidence for this syndrome in humans,” said Dr. Shanna Swan, the study’s lead researcher and a professor in the Department of Obstetrics and Gynecology at the University of Rochester School of Medicine and Dentistry. “Animals [exposed to phthalates] definitely have decreased testosterone, so it is likely that this is happening in humans too.”

The study is the strongest evidence yet that manmade chemicals in the environment can feminize male babies in the womb.

Yet scientists say a larger study of babies should be conducted, and they should be followed into adulthood to see whether they develop low sperm counts or any other reproductive problems.

“It’s such an important observation, you’d like to see this done again, with more children and another population,” said Earl Gray, a U.S. Environmental Protection Agency reproductive toxicologist whose research has found that phthalates feminize male rodents.

The Los Angeles Times is a Tribune Publishing newspaper.