And, it's more likely to happen here in the U.S. than anywhere else in the world.

Susan Hunt doesn't want to, but she remembers the day well.

She got a call from her doctor that her mammogram test showed possible cancer.

"It's incredibly stressful... it's very hard on family members," she said.

She went in for more tests and a complicated biopsy. But in the end, she had no evidence of malignancy.

"It's very common... sooner or later most women experience it," said Susan's doctor Joann Elmore.

Elmore noticed a shocking number of women who get false-positive results. She says if women get annual mammograms for 10 years, one out of every two women will get at least one false-positive result.

Even more surprising: Her research shows women in North America are two to four percent more likely to get a false-positive test than anywhere else in the world.

"Screening mammograms aren't perfect... we wish they were," Dr. Elmore sai.

But Dr. Elmore doesn't think the mammograms are the reason for false positive results. She says it's probably the people who read the tests.

"Radiologists, I think, are understandably anxious about missing a cancer," she said.

And missing cancer is the number one reason people file malpractice claims.

Dr. Elmore says it's still critical for women to get mammograms, and so does Susan. She says she has not lost faith in the test -- she'd rather be sure.

"A false positive in my mind beats a false negative anytime."

The high number of false-positive tests did not result in North American doctors diagnosing cancer more often.