Mammograms sometimes find cancers that would have gone away on their own, researchers say.

A study in four Norwegian countries found that cancer rates went up when women started getting scans every two years. The rates stayed higher among women who got checked that often than in those who were screened only once every six years.

The authors said that if all of the detected cancers would have shown up later in other ways -- once they started affecting a woman's life -- than a fall in cancer rates should have been seen.

The large study looked at nearly 230,000 women ages 50 to 65.

"It appears that some breast cancers detected by repeated mammographic screening would not persist to be detectable by a single mammogram at the end of six years," wrote Dr. Per-Henrik Zahl of the Norwegian Institute of Public Health. "This raises the possibility that the natural course of some screen-detected invasive breast cancers is to spontaneously regress."

The findings do not answer the question of whether mammograms prevent deaths from breast cancer, the authors noted.

Screenings that detect cancers that never would have harmed a patient are a concern because treatment for cancer, or any other disease, carries risks from side effects, as well as financial costs.