Sperm counts are used to measure infertility
Others may not have got the help they needed to conceive.
Scientists have said the standard measurements used by doctors for decades to assess men's fertility may have been flawed.
Doctors currently use World Health Organisation (WHO) standards to assess how fertile semen samples are.
The new measurements suggested by them both raise the threshold for fertility and lower the threshold for infertility - with a "grey" area in the middle.
Dr Duane Alexander, director of National Institute of Child Health and Human Development which funded the research, said: "These recommendations provide a more reliable means for estimating the likelihood of a man's fertility.
"This will prove a valuable tool for specialists treating couples with unexplained infertility, allowing them to make their best judgement on whether it would be more helpful to focus on treatment for the man or the woman."
The authors estimate one in six couples have problems conceiving.
In 30 - 40% of these cases, it is the man who experiences fertility problems.
The test to determine male infertility is an analysis of the amount of sperm in semen, how many are moving, and how many are of the uniform oval shape.
Sperm counts
The US team, from the University of Rochester Medical Center, studied the sperm quality of 1,500 men - 765 from infertile couples and 696 from fertile couples.
The Rochester team say the shape and appearance of sperm is an important factor in determining which men are fertile, but say the WHO guidelines fail to refer to it at all.
Their research defined new categories doctors could use as reference points:
Dr David Guzick, chairman of obstetrics and gynaecology at the University of Rochester Medical Center said: "Every treatment for infertility depends upon first establishing what's normal and abnormal.
"Up until now, we've just been using guidelines without rigorously testing them.
"We hope specialists will use these revisions in their counselling of infertile couples and in tailoring treatments to individual patient circumstances."
'No surprise'
Fertility expert Dr Simon Fishel, director of Centres for Assisted Reproduction in the UK said the US recommendations did not come as any surprise to him.
He cited research which had shown men who had fathered children naturally had sperm counts which, had they gone for tests, would have indicated they needed high-tech fertility treatment.
Dr Fishel said some GPs, obstetricians and gynaecologists often stuck too rigidly to guidelines. meaning patients were given fertility treatment when they could have conceived naturally - and others who could have benefited from such treatment did not receive it.
"The most important thing is to use the guidelines as such. They should be used as guidelines not gospel," he said.