Gender measure: pollutant appears to alter boys' genitals.

Infant boys who were exposed in the womb to modest concentrations of certain common plasticizers and solvents developed genital changes including smaller-than-normal penises, a new study finds.

The results from this study of 85 boys are consistent with what researchers have seen in laboratory animals treated with the chemicals, which are called phthalates. In such tests, the substances impair fetal production of testosterone and other male sex hormones (SN: 4/3/99, p. 213).

In the current study, mothers with the highest phthalate exposures bore boys with slightly less space between the gonads and anus than did mothers with less phthalate exposure, as gauged by the women's urine concentrations of the chemicals during pregnancy. Moreover, boys with a short anogenital distance tended to have smaller penises and were far more likely to have testes that didn't descend properly into the scrotum.

Anogenital distance is typically longer in males than in females. In rodents, prenatal phthalate exposure can erase this difference, says Paul M.D. Foster of the National Institute of Environmental Health Sciences in Research Triangle Park, N.C. He's shown that this shortening in male animals can signal major permanent impairments in reproductive organs (SN: 9/2/00, p. 152).

In people, the anogenital distance "is not a measurement that is typically made," notes J. Bruce Redmon, an endocrinologist at the University of Minnesota Medical School in Minneapolis and an investigator in the new study.

In their research, he and his colleagues linked the signs of demasculinization to far lower phthalate exposures than have previous studies involving animals, notes study leader Shanua Swan of the University of Rochester (N.Y.) School of Medicine and Dentistry. Other research has already shown that more than one-quarter of U.S. women have phthalate concentrations in their bodies greater than those deemed in the new study to have genital-altering effects.

The data from boys suggest that in people, phthalates "may reduce the production of testosterone by the fetal testis, with subsequent downstream consequences," says Richard M. Sharpe of the University of Edinburgh's Centre for Reproductive Biology in Scotland. Such effects include low sperm production in adulthood and increased risk of testicular cancer, he says.

For the new report, Swan and her colleagues regularly examined pregnant women in several cities until the births of their babies. The researchers made a battery of measurements on the babies shortly after birth (SN: 11/13/04, p. 318).

The anogenital distances for boys exposed to the highest amounts of phthalates were 19 percent shorter, on average, than those of boys less exposed. Although that's not a large enough difference for most doctors to notice, Sharpe says, the finding is "potentially, a big deal."

The study results, which will appear in an upcoming Environmental Health Perspectives, show a statistically strong association between a short anogenital distance and high exposures to four phthalates. These chemicals were diethyl phthalate, used in perfumes; dibutyl phthalate, used in nail polish and hard plastics; butyl benzyl phthalate, used primarily in flooring; and diisobutyl phthalate, used in paints, adhesives, and polyvinyl chloride plastics.

Referring to the new study, Marian K. Stanley of the chemical industry's Phthalate Esters Panel observes, "It's hard to know what to make of it." She says that there's no definition of a normal anogenital distance in babies. Stanley also observes that diethyl phthalate, which the new study links to shortened anogenital distances, has no such effect in lab animals.

Although Redmon agrees that the norm for boys is unknown, he adds that a reduced anogenital distance "is what you would expect" if phthalates lower the concentrations of male hormones in the human fetus.

Regarding diethyl phthalate, Swan notes that several studies have linked the chemical to reproductive effects in people.

Stanley, Redmon, and others agree that the infants in the current study should be followed for several years. They also call for studies based on similar measurements in larger groups of pregnant women and their infants.