A new study suggests mammograms don't help women over 50 as much as previously thought.

Researchers in Norway found that breast cancer screening can only be credited for a third of the reduced risk of death. The rest is due to better treatment and greater awareness of the disease.

The study is published in the New England Journal of Medicine.

"It's not the great lifesaver that people think it is. It's not a magic bullet," said Georgetown University researcher Dr. Jeanne Mandelblatt, who was not involved in the study.

Still, the American Cancer Society stands firmly by its recommendation that women get yearly mammograms beginning at age 40.

The group says early detection is key to surviving breast cancer and mammograms are a first line of defense.