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Men's obsession with size in their nether region may be justified after all. New research indicates size — of a sort — does matter when it comes to male fertility.

It turns out it's not penis size that's significant, but the length as measured from the anus to underneath the scrotum, known as anogenital distance, or AGD, according to research published this month in the journal Environmental Health Perspectives.

Fertility testing is nothing if not invasive, but the research, from the University of Rochester Medical Center, means that the latest tool in the fertility arsenal might soon be a lowly tape measure.

AGD is associated with semen volume and sperm count, according to study author Shanna Swan, a reproductive epidemiologist and professor in obstetrics and gynecology at the University of Rochester.

The median AGD length is about 2 inches; men with a shorter measurement stand a seven-times greater risk of having fertility problems as opposed to men with a longer AGD. They are more likely to be sub-fertile, which generally indicates a sperm count of less than 20 million per milliliter. Men with sperm counts in this range are only half as likely to succeed in getting a partner pregnant as men with more typical sperm counts in the range of 50 to 60 million sperm per milliliter.

The researchers, who measured the AGDs of 126 college students who were not attempting to get a partner pregnant, did not attempt to analyze why some men had shorter AGDs than others. But earlier research has raised the possibility that pregnant women exposed to a controversial class of chemicals called phthalates — used in perfumes and personal care products including shampoos, as well as PVC-infused products such as shower curtains and floor coverings — may give birth to boys with shorter AGDs.

In 2005 and again in 2008, Swan found that mothers exposed to high levels of phthalates during pregnancy bore baby boys with reduced AGD and penis size. In previous animal studies, shorter-than-average AGD in male rodents has corresponded with eventual fertility problems.

"We're not at all saying chemicals are the only explanation, but we found when women are exposed to common phthalates that decrease testosterone, there is a whole cascade of things that can happen," says Swan. "The short AGD is not happening in isolation."

Not everyone is quick to embrace the linkage. "Assessment of AGD as a routine evaluation of one's fertility is premature," Natan Bar-Chama, who heads the male reproductive medicine at Mt. Sinai Hospital in New York City, told Reuters.

Swan acknowledges that her study needs to be replicated, but she believes her current research, along with previous data from animals and humans, suggests AGD is a "reliable marker" of potential infertility.

When combined with sperm count analysis, AGD is not a diagnosis in and of itself, but it can be useful test in terms of providing additional information to doctors and couples.

It's also easy to administer. No pokes and prods needed; just a simple measurement that's so easy, couples having trouble getting pregnant could conceivably do it at home.

Which may or may not be a good idea. "I'm not going to recommend that people do it yet," says Swan, "but I suspect it's happening."

• Bonnie Rochman