If you're born with only one kidney, you can forget about being a football star or a hockey hall-of-famer - or at least, that's been the conventional medical wisdom espoused for the last few decades.

But a new study found that missing a kidney shouldn't necessarily keep kids from playing contact sports.

Researchers at the University of Utah combed data on high school athletes' injuries reported to the National Athletic Trainers' Association, including those playing collision-prone sports like football, field hockey and basketball. Of the more than 23,000 injuries that players sustained from 1995 to 1997, just 18 were kidney injuries. And even those injuries were fairly mild. None of the players had catastrophic kidney injuries that required major medical care.

Injuries to knees, eyes and heads were much more common. Among football players, there were 64 concussions for every one kidney injury.

Dr. Matthew Grinsell, the study's lead author and an assistant professor of pediatrics at the University of Utah School of Medicine, said based on the numbers, a lot of other everyday activities are riskier for the kidneys than playing contact sports.

"It looks like bicycle riding and downhill skiing are more dangerous than football," he said. "And all sports are way down the list of risks compared with motor vehicle accidents."

About one in 750 people are born without one of their kidneys, according to the National Kidney Foundation. Others are born with a kidney that simply doesn't work or needs to be removed because of a tumor or an abnormality in the urinary tract.

In the past, doctors have erred on the side of caution when it comes to people who have only one of an organ that usually comes in pairs, like kidneys or testicles. Better to avoid anything that might put the remaining organ at risk, like crashing into a linebacker.

But Dr. Alex Diamond, an assistant professor of pediatrics and a team physician for Vanderbilt University, said doctors usually try to strike a balance between protecting vital organs and reaping the benefits of athletics when advising patients and their families.

"You don't want to underrestrict someone who could be dangerously injured, but you don't want to overrestrict someone from all the social, physical and psychological benefits of playing sports," he said.

The American Academy of Pediatrics doesn't say that all children who are down one kidney should be kept on the sidelines. Instead, the group recommends giving these kids a "qualified yes" for sports participation after they are examined and cleared for play by a doctor.

"We have good supporting evidence that we can maybe start letting some more of these kids play," Diamond said. "And this study certainly helps."

Grinsell noted that it's not possible to tell from the NATA's data if any of the players reporting injuries had only one kidney. Most likely, they had two. He also said only children and teens with a kidney that functions normally and is in the normal location in the body should be cleared for contact sports. Kids who have a transplanted kidney should probably steer clear of collision sports.

And the risk of a sports-injured kidney may pale in comparison to the risks to the kidneys and other organs posed by obesity, diabetes and other chronic diseases linked to inactivity.

"Trauma from sports and being active is not as big a risk factor for kidney disease and needing a kidney transplant as a sedentary, unhealthy lifestyle," Grinsell said.