The dimension in question is not penis or testicle size, but a measurement known as anogenital distance, or AGD.
Men whose AGD is shorter than the median length -- around 2 inches (52 mm) -- have seven times the chance of being sub-fertile as those with a longer AGD, according to a study published on Friday in the journal Environmental Health Perspectives.
That distance, measured from the anus to the underside of the scrotum, is linked to male fertility, including semen volume and sperm count, the study found. The shorter the AGD, the more likely a man was to have a low sperm count.
This offers the prospect of a relatively simple screening test for men, said study co-author Shanna Swan of the University of Rochester Medical Center.
"It's non-invasive and anybody can do it, and it's not sensitive to the kinds of things that sperm count is sensitive to, like stress or whether you have a cold or whether it's hot out," Swan said in a telephone interview.
"If somebody's got a short AGD, particularly if they have problems conceiving, I would say get to the infertility doctor, because the chances are good that something is wrong."
To reach their conclusions, researchers measured the AGDs of 126 men born in or after 1988, a small but statistically significant sample, Swan said.
The study did not address what might cause certain men to have short AGD measurements.
But previous studies, published in 2005 and 2008, looked at the possible link between mothers who were exposed to chemicals called phthalates during pregnancy and the AGD of their infant and toddler sons.
Phthalates are a group of chemicals widely used in industrial and personal care products, including fragrances, shampoos, soaps, plastics, paints and some pesticides.
In these earlier studies, the scientists tested for the presence of phthalates in the pregnant women's urine. They found that women who had high levels of phthalates in their urine during pregnancy gave birth to sons who were 10 times more likely to have shorter than expected AGDs.
Swan, who also co-authored the earlier papers, said they showed the correlation between prenatal phthalate exposure and shorter AGD.
The latest study does not address prenatal phthalate exposure directly, "but it does answer the question of why we should care about AGD," Swan said. "And it does suggest that whatever is altering AGD is also altering sperm count."
Key genital measurement linked to male fertility
When it comes to male fertility, it turns out that size does matter
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