Hypospadias, one of the most common birth defects among baby boys, apparently is not increasing in the U.S., casting doubt on whether boys are harmed by phthalates and other endocrine-disrupting chemicals thought to trigger reproductive abnormalities.

Researchers have reported that the hypospadias rate stayed the same in New York State between 1992 and 2005. An earlier study also found no increase in California boys between 1984 and 1997.

Hypospadias, a condition in which the urethra opening is on the underside of the penis rather than the tip, occurs in roughly one of every 250 male births. Surgery is normally required or the condition can lead to infertility.

Some environmental scientists have suspected, based on studies of lab animals, that exposure to chemicals that block testosterone may be partially responsible for the birth defects.

In tests of lab rats, chemicals called phthalates, which are widely used in plastic and personal care products, cause several reproductive abnormalities that scientists have dubbed “testicular dysgenesis syndrome" or "phthalate syndrome." Included are hypospadias, undescended testes, reduced sperm counts and testicular cancer.

In the new study, New York Presbyterian Hospital urologists said that because hypospadias rates are stable, it casts doubt on whether human boys are harmed by phthalates or other endocrine-disrupting chemicals.

The researchers reported in the journal Urology that “these data suggest that the testicular dysgenesis syndrome described in animal models may not be evident in humans.” The lead author was Dr. Harry Fisch, director of the hospital’s Male Reproductive Center and a professor of clinical urology at Columbia University.

But other experts say the study does not invalidate the theory.

“The evidence seems to suggest that there hasn't been a big increase in hypospadias over recent years, which does weaken the argument that new endocrine disruptors in the environment are causing hypospadias,” said Kim Harley, a University of California, Berkeley epidemiologist who studies environmental exposures and human health but was not involved with the study.

Nevertheless, she added that it doesn’t rule out that phthalates or other chemicals have a role in causing the defects. Looking at how birth defect rates change over time is not an adequate way of examining environmental connections.

“Hypothetically, what if there were something that we were exposed to in the past that also was associated with hypospadias? If that other exposure were decreasing at the same time as phthalates were increasing, it might wash out the effect and make rates stable over time,” Harley said. “That is why a study like this can only hypothesize based on time trends, but can’t make a one-to-one link between a chemical and a disease.”

Russ Hauser, a Harvard School of Public Health professor of environmental and occupational epidemiology who studies phthalates and other endocrine-disrupting chemicals, agreed. “There are many reasons for why hypospadias may or may not have changed over time. Other risk factors also change over time," he said.

The cause of hypospadias is unknown, but medical experts suspect that something in the womb disrupts genes that regulate male hormones. A variety of factors, including the mother’s age, could be to blame. The New York study found that hypospadias is more common in boys born to mothers 35 years or older.

Although hypospadias may not be increasing now, several reports previously noted a doubling of rates in the 1970s and 1980s.

Traces of many phthalates have been found in the urine of nearly every human tested. One study, conducted in Germany, found that some of the compounds have increased in people over the past 20 years, while others have decreased, and that exposure varies greatly from place to place. Some people exceed the daily levels that the U.S. Environmental Protection Agency or European Food Safety Authority consider safe.

But whether the chemicals are harming people, particularly boys, has become quite controversial in the past couple years.

Chemical industry representatives have pointed to the hypospadias study as evidence that phthalates, which have been used as a plasticizer in vinyl for about 50 years, are not causing male reproductive abnormalities. In addition, while some studies have found that men’s sperm counts have declined substantially, others have reported no decline, which also casts doubt on the human effects of endocrine disruptors.

But Hauser said the hypospadias study does not invalidate the testicular dysgenesis theory because no data was collected on the boys’ exposure to phthalates or other chemicals.

Researchers would need to collect exposure information on a large number of pregnant women, and then see if the rate of hypospadias among the highly exposed boys is different than those with low exposure, Hauser and Harley said.

The findings are useful for exploring trends in hypospadias over time, not exploring potential causes, they said.

“Over-interpretation should be avoided, especially using them to try to prove a negative,” Hauser said.

In addition to animal studies, other research has linked phthalates to reproductive effects.

Hauser and his colleagues reported in 2006 that men with higher levels of one common phthalate had lower sperm concentrations and quality. And a study of 176 male infants by University of Rochester scientists found that babies exposed to higher amounts of phthalates had a shorter anogenital index (the length of the perineum), which is a sign of feminized hormones.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.