In a study that questions the current standards for diagnosing male infertility, US researchers have found that sperm shape may be most telling of a man’s ability to have children.

The investigators also uncovered new “thresholds” for sperm count and mobility that distinguish fertile and subfertile men.

The findings call into question widely used standards for determining male fertility established by the World Health Organization (WHO), according to a report in the November 8th issue of The New England Journal of Medicine.

“Every treatment for infertility depends upon first establishing what’s normal and abnormal,” lead study author Dr. David S. Guzick said in a statement. “Up until now, we’ve just been using guidelines without rigorously testing them.”

Guzick, of the University of Rochester in New York, and his colleagues studied semen samples from the male partner in 765 infertile couples and 696 fertile couples seen at nine US hospitals. The researchers were looking for key features that determine a man’s fertility: the number of sperm per milliliter of semen, the percentage of sperm that are moving (sperm motility) and the shape of the sperm.

Current WHO standards state that “normal” semen contains 20 million sperm per milliliter, with at least 50% motility. They do not provide a norm for sperm shape.

Yet Guzick’s team discovered that shapely sperm–those that have, among other things, an oval-shaped head–were important in a man’s fertility.

In fact, they report, “sperm (shape), as measured according to strict criteria, appears to be the most informative measurement for discriminating between fertile and infertile men.” The researchers add, however, that none of the three criteria alone was enough to make a diagnosis.

Guzick and his colleagues also came up with new values for sperm count and motility. Instead of designating men as “normal” or “abnormal,” they describe fertility as a sperm count of 48 million per milliliter or more, with motility of 63% or more and at least 12% having a normal appearance. Subfertility includes a sperm count of 13.5 million per milliliter or fewer, a motility of less than 32% and fewer than 9% of sperm having normal features. The researchers designate anything between these values as “indeterminate” fertility.

All of this means that men who were formerly considered fertile might instead have difficulty fathering a child, while at least some of those who fall below the current fertility standards may be able to father children, according to the National Institute of Child Health and Human Development (NICHD). The Bethesda, Maryland-based federal agency funded the study.

“Confirmation of the validity of these thresholds for semen measurements in an independent sample of fertile and infertile men is needed,” Guzick’s team points out.

“Nevertheless,” they note, “our results do provide a reference value for sperm (shape) that is missing from the current WHO manual for semen evaluation.”

“These recommendations provide a more reliable means for estimating the likelihood of a man’s fertility,” the NICHD’s Dr. Duane Alexander said in a statement. “This will provide a valuable tool for specialists treating couples with unexplained infertility.”