Shapely sperm is the best indicator of a man's fertility, according to a study that rewrites the standards for analyzing semen samples.
Current standards under- or overestimate many men's fertility, said the study, published in Thursday's New England Journal of Medicine.
``Every treatment for infertility depends upon first establishing what's normal and abnormal,'' said Dr. David Guzick, chairman of obstetrics and gynecology at the University of Rochester Medical Center. ``Up until now, we've just been using guidelines without rigorously testing them.''
The World Health Organization's widely used standards classify semen as normal if it contains 20 million sperm per milliliter and at least half of them are moving around. But the cut-off points for who is fertile and who is sterile are not particularly clear.
And the organization's criteria for the percentage of well-shaped sperm have changed over the years, Guzick said: ``It was 50 percent, then 30 percent, and now an asterisk. They basically say, `We really don't know what it is.'''
Guzick and doctors at seven other universities studied sperm from the men in 765 infertile couples and 696 couples who had children.
His study found that men were most likely to be fertile if their semen had more than 48 million sperm per milliliter, more than 63 percent of them were moving, and more than 12 percent of them looked normal.
They were likely to be infertile if there were fewer than 13.5 million sperm per milliliter, fewer than 32 percent were moving and fewer than 9 percent were well-formed.
For measurements in between, he said, ``the best you can say is `maybe, maybe not.'''
A well-shaped sperm has, among other things, an oval head and whippy tail. Circular heads, enlarged heads and tails that are scrunched up are some of the more easily spotted problems.
Although the 3 percent difference between the number of shapely sperm in fertile and infertile men's semen seems small, it was the most reliable indicator, Guzik said.
Dr. Duane Alexander, director of the National Institute of Child Health and Human Development, which sponsored the study, said the recommendations will be a valuable tool for infertility specialists.
About one in six couples has trouble conceiving. In one-third or more of those cases, it is the man who is infertile, Guzick said.
He noted that none of the measurements was infallible: A few men had children even though their semen was in the ``subfertile'' range, and some men had been unable to father children even though their semen looked fine.