FRIDAY, March 4 (HealthDay News) -- New research contends that the distance between the posterior base of the scrotum and the anus can predict the strength of a man's sperm population.

Men who have a shorter perineal length, also known as the anogenital distance (AGD), have lower sperm counts, poorer quality sperm, lower sperm concentrations and lower motility, the study has found.

Although scientists had long ago made the connection between AGD and fertility in male rats, this is the first time it's been shown in humans, said the researchers, whose findings are published in the March 4 edition of the journal Environmental Health Perspectives.

In rats, the same group of researchers established that shorter AGD and male reproductive problems were linked to exposure in the womb to a class of endocrine-disrupting chemicals called phthalates, which are used to make plastics more flexible and are found in many toys and other household products.

Phthalates disrupt normal testosterone exposure in the womb, among other things, and have been linked to fertility problems in women and abnormal breast growth in boys.

The new study did not examine phthalate exposure in the subjects or their mothers, but by making a connection between shorter AGD length (linked in male rats to exposure to phthalates) and lower sperm counts, it does provide indirect evidence that exposure to such chemicals may result in lower sperm counts in adult men, according to the authors.

The researchers took two measures of AGD, along with sperm count and concentrations, sperm shape and sperm motility, in 126 college students in upstate New York.

The first measure was from the underside of the scrotum to the anus and the second was from the top of the penis stem to the anus.

The shorter measurement -- starting at the underside of the scrotum -- was associated with sperm count, sperm concentration and motility. The longer measurement was not.

The median AGD was about 52 millimeters so men who had an AGD below this level were more than seven times more likely to have low sperm concentration in the subfertile range.

Twenty-five percent of the sample "had sperm concentrations that would have sent them to a doctor for an infertility work-up," said study senior author Shanna H. Swan, professor and associate chair of obstetrics and gynecology at the University of Rochester Medical Center.

Biologically speaking, the shorter AGD reflects lower exposure to testosterone in the womb, which can affect full development of the genital tract in males, affecting both this size measurement and sperm quality, she explained.

Swan and the other researchers have noted that an association between the volunteers' lower sperm counts and phthalates exposure in the womb is "speculative," but one that warrants further investigation.

Meanwhile, in fertility screening, AGD may prove to have advantages over other factors that affect sperm count. Unlike other variables that affect semen quality, such as stress, having had a fever recently and even outside temperature, "AGD is forever," Swan said, making any predictions more accurate.

And if the sperm count is merely low (as opposed to non-existent), taking out a ruler and measuring may "help the prediction of whether he's going to be successful [getting a woman pregnant]," Swan said.

But, she stressed, "we need a lot more men to get normative values. We need to have a couple of thousand to say what you would expect in the general population."

One expert agreed that more research was needed to determine AGD's possible use as a marker for fertility problems in healthy men.

"We don't have good ways to predict future fertility unless there are major abnormalities," said Dr. Sheela Sathyanarayana, assistant professor of pediatrics at the University of Washington and Seattle Children's Hospital, who was not involved in the study.

"This could potentially be used in the future as a marker for sperm concentration and motility," Sathyanarayana said, "[but] more research groups would need to trial this out and look to see if there are differences in AGD between men who are fertile and men who are infertile."

More information

The U.S. National Institute of Child Health and Human Development has more about fertility and infertility.