The controversy over when a woman should get mammograms is about to heat up again.

The U.S. Preventive Services Task Force, an independent panel of experts, whose members are appointed by the federal government, issued a final set of recommendations late Monday saying that women between the ages of 50 and 74 should get routine screening once every two years.

The task force’s guidelines are important because insurers and government programs often follow the panel’s recommendations in deciding whether to cover certain preventive services.

The task force’s final recommendation is likely to be controversial because some other groups say the screening should start earlier. The American Congress of Obstetricians and Gynecologists, for example, recommends that regular screenings begin at age 40, while the American Cancer Society calls for women to start yearly screening at age 45 and then move to screening every two years starting at age 55.

Congress has sided with proponents of earlier screening. Last month, in anticipation of Monday’s release of the task force’s final recommendation, lawmakers took pre-emptive action: It directed insurers to ignore the task force’s latest guidelines and, instead, to rely on its 2002 recommendation. That called for annual mammograms to begin at 40. As a result of the congressional action, women in their 40s will continue to be able to get annual mammograms at no cost.

The differences over when to start regular screening reflect the growing concern that the benefits of mammograms may have been oversold, and that they don’t outweigh the anxiety and potential harm caused by over-diagnosis and false positives from the tests.

The debate over when to start regular screening involves only women of “average risk” who don’t have specific risk factors for breast cancer such as the BRCA1 and BRCA2 genetic mutations or a family history of the disease. They’re also not aimed at diagnostic mammography, which takes place once a woman has a symptom such as a lump.

In releasing its final recommendations, the task force confirmed an earlier guidance it issued that said screening mammography had the greatest benefit for women ages 50 to 74. For women in their 40s, the likely benefit is less and the potential harms are proportionally greater, it said. The most serious potential harm is unneeded treatment for a type of cancer that would not have become a threat to a woman’s health.

The congressional action, which was included in the recently enacted spending law, has drawn criticism from some experts.

“The U.S. Congress thinks it’s perfectly acceptable, even preferred, for a scientific document from 14 years ago to guide coverage policy on screening for breast cancer in women,” says Kenneth Lin, a Georgetown University family medicine doctor who teaches preventive and evidence-based medicine.