March 9, 2011 -- Men whose anus-to-scrotum distance is shorter than average are 7.3 times more likely to have low sperm counts than men with a longer anogenital distance.

The finding comes from Shanna Swan, PhD, of the University of Rochester, N.Y., and colleagues. In earlier studies, Swan found that pregnant animals exposed to phthalates -- commonly used chemicals suspected of hormone-disrupting effects -- had infertile male offspring with short anogenital distance.

In a 2005 study, Swan's team found that male infants with prenatal exposure to phthalates had decreased anogenital distance.

In their current study, the researchers found poor sperm quality in men with short anus-to-scrotum distance. They tended to have low sperm counts, low sperm motility, low sperm concentration, poor sperm morphology, and low total mobile sperm count.

"The associations we observed between these sperm parameters and anogenital distance were stronger than those for most [other factors] known to be associated with semen quality," Swan and colleagues note. "If our results are confirmed, anogenital distance may provide a useful adjunct to these traditional measures of male reproductive function."

Swan's team studied 126 young men (most were 19 years old) who volunteered for the Rochester Young Men's Study. Their anogenital distance was measured in two ways: from the center of the anus to the base of the scrotum, or from the center of the anus to the top of the penis at the point where it intersects with the abdomen. Only the scrotal measure turned out to be linked to subpar fertility.

One surprise from the study was that nearly a fourth of the young men in the study had low sperm counts.

Swan and colleagues did not report on whether the men's mothers reported exposure to phthalates during pregnancy, and they did not report data on the men's hormone levels.

"Poorer semen quality and shorter anogenital distance in adulthood may reflect a common origin, including a disruption of testicular development in utero," they suggest.

Although there may be many reasons for this, they note, it "may be caused by exposure to endocrine-disrupting chemicals" in the womb.

Swan and colleagues report their findings in the March 4 online edition of Environmental Health Perspectives.