Fewer U.S. Women Get Breast Cancer Test

"One possibility is women who stop using HRT [hormone replacement therapy] believe their risk of breast cancer has declined and so they don't feel the need for mammography," Breen said. "Another possibility is they may not be going to their doctors as often for their hormones, and so are not being told it's time for a mammogram. That's a concern because just because they are not taking HRT does not mean they are not at risk."

Another possible explanation is that a worsening shortage of mammography facilities makes it more frustrating to get an appointment.

"It can be very difficult," said Carol J. Fabian, a breast cancer specialist at the University of Kansas. "You can wait months. I think that's a big part of it."

Breen and others speculated that some of the drop may be due to the debate over the benefits of mammography. Some experts question the blanket recommendation that women routinely undergo mammograms starting at age 40, saying the risks of unnecessary treatment and anxiety from false alarms may outweigh the benefits for some women at low risk.

"Some women are beginning to balance the risks and the benefits," said Carolina Hinestrosa of the National Breast Cancer Coalition, an advocacy group. "If women are making a careful determination and an informed decision after weighing the risks and benefits, I don't necessarily think that's a bad thing."

But others argued that the benefits have been well established, particularly for women over age 50. Even though other methods, such as MRIs, are showing promise, mammograms remain the most effective widespread screening method.

"Too many women and clinicians are misunderstanding the 'controversy' about mammography," said Constance Lehman, a professor of radiology at the University of Washington. "A mammogram is the best thing a woman can do to reduce her chances of dying from breast cancer."

Another possible explanation is that the rising survival rates among breast cancer patients are making women more complacent.

"Women may also be feeling, 'Well, the death rates are dropping in the population so I don't need to get screened,' " Breen said. "That's kind of missing the point. One reason death rates are dropping is because screening rates were so high."