A new study questions the value of mammograms for breast cancer screening. It concludes that a woman is more likely to be diagnosed with a small tumor that is not destined to grow than she is to have a true problem spotted early.

Mammograms do catch deadly cancers and save lives. But they also find many early cancers that will never threaten health. Treatment has improved so much over the years that finding cancer early has become less important.

The study tracked tumor sizes before and after mammograms came into wide use, and what effect screening had on death rates. They conclude that two-thirds of the drop in mortality was due to better treatments, not early detection.

The study was published Wednesday by the New England Journal of Medicine.