Women whose screening mammograms produce false alarms have a heightened risk of being diagnosed with breast cancer years later, but the reason remains mysterious, researchers say.

An increased risk of breast cancer among women with a “false-positive” mammogram has been reported before.

What’s new about this study is that the authors tried to figure out how much, if any, of the extra risk is due to doctors missing the cancer the first time they investigated the worrisome mammogram findings.

But mistakes from doctors missing cancers explained only a small percentage of the increased risk, according to lead author My Catarina von Euler-Chelpin, an epidemiologist from the University of Copenhagen in Denmark.

She said that she could not explain most of the increased risk of later breast cancer in women with false-positive mammograms. (A mammogram is considered false positive when it suggests possible breast cancer but additional screenings or a biopsy fails to find it.)

Of the more than 58,000 Danish women who had mammography between 1991 and 2005, her study identified 4,743 women with suspicious findings that were eventually declared negative.

By 2008, 295 of those 4,743 women had been diagnosed with breast cancer, von Euler-Chelpin recently reported in Cancer Epidemiology.