Mammogram rates in the U.S. have declined by nearly 6% among women in their 40s since screening guidelines were revised in 2009, according to a new study.

"This represents a small but significant decrease," say the Mayo Clinic researchers, because the guideline changes were controversial when they were released.

With the revised guidelines, the U.S. Preventive Services Task Force (USPSTF) called for mammograms to begin for women at average risk at age 50 and occur every two years until age 74. This was a change from its 2002 recommendations, which called for women to have mammograms every one to two years starting at age 40.

But the task force health experts concluded the benefits of screening women 40 to 49 did not outweigh its harms. They also believed there was not enough evidence to suggest that mammograms were effective for women 75 and older.

In contrast, the American Cancer Society guidelines call for yearly mammograms for women at average risk starting at age 40 and continuing for as long as a woman is in good health. Unlike the USPSTF, they do not recommend an age cut-off for the screening test.

"The 2009 USPSTF guidelines resulted in significant backlash among patients, physicians, and other organizations, prompting many medical societies to release opposing guidelines," study researcher Nilay Shah, PhD, says in a news release. He is a researcher at the Mayo Clinic Center for the Science of Health Care Delivery in Rochester, Minn.

The new findings were presented this week at the Academy Health Annual Research meeting in Orlando.

Slideshow: Essential Screening Tests Every Woman Needs
Modest Drop in Younger Women

Researchers wanted to find out what impact the new mammogram guidelines -- and the debate over them -- had on the screening rates in younger women.

For this large study, they analyzed health insurance claims data from nearly 8 million women aged 40 to 64 nationwide. They tracked women who had mammograms between January 2006 and December 2010, a time period before and after the screening guidelines were updated.

When researchers compared the mammogram rates before and after the new changes, they found a nearly 6% drop in screenings in women 40 to 49. They estimate this meant that about 54,000 fewer mammograms were done in this age group one year after the guidelines were revised, which they describe as a "modest effect."

"A modest effect is also in line with the public resistance to the guideline change and the subsequent release of numerous conflicting guidelines," the researchers write.

During the same time frame, the changes were shown to have no effect on mammogram rates in women 50 to 64. Researchers suggest this reflects a more subtle change in the screening guidelines, which went from yearly to every two years in this age group, compared to a "more radical change" in younger women.

This study was presented at a medical conference. The findings should be considered preliminary, as they have not yet undergone the "peer review" process, in which outside experts scrutinize the data prior to publication in a medical journal.