The cancer group also now advises switching to screening to every other year at 55. The task force recommends starting routine screening for breast cancer at age 50, then every other year.

It’s not a one-size-fits-all recommendation; both groups say women’s preferences for when to be scanned should be considered.

The advice is for women at average risk for breast cancer. Doctors generally recommend more intensive screening for higher-risk women.

“The most important message of all is that a mammogram is the most effective thing that a woman can do to reduce her chance of dying from breast cancer,” said Dr. Richard Wender, the cancer society’s cancer control chief.

“It’s not that mammograms are ineffective in younger women,” he said, but at age 40, breast cancer is uncommon and false alarms are more likely.

Concern about false alarms contributed to the cancer society’s new guidance. These lead to worry and more testing — they mean an initial result was suspicious but that cancer was ruled out by additional scans and sometimes biopsies.

The latest guidelines acknowledge that some younger women are willing to accept that, and that for them starting annual exams at age 40 is fine, as long as they know the risks.

The guidelines were developed by experts who reviewed dozens of studies including research published since 1997 — the year the trusted medical group recommended yearly mammograms starting at age 40, and since 2003, when it stopped recommending monthly breast self-exams.

The update also drops a recommendation for routine physical breast exams by doctors, saying there’s no evidence that these save lives.

The Rev. Jennifer Munroe-Nathans, a church pastor in Millis, Massachusetts, said she hasn’t paid attention to guidelines and started getting annual scans around age 40 on her doctor’s advice. Her mother had breast cancer; so have some of her congregants and Munroe-Nathans said she has no plans to change course when she gets older.

“For my own peace of mind I intend to continue yearly mammograms,” she said. “I’ve seen the impact of breast cancer — perhaps that makes me a little more hyper-vigilant.”

The society’s updated guidelines say switching to every other year at age 55 makes sense because tumors in women after menopause tend to grow more slowly. Also, older women’s breasts are usually less dense so cancer is more visible on mammograms, said Dr. Kevin Oeffinger, chairman of the society’s breast cancer guideline panel and director of the cancer survivorship center at Memorial Sloan Kettering Cancer Center in New York.

The guidelines were published Tuesday in the Journal of the American Medical Association.

More than 200,000 women in the U.S. are diagnosed each year with breast cancer and about 40,000 die from the disease. Overall, 1 in 8 women will be diagnosed with the disease at some point and chances increase with age.

Oeffinger said women need to be familiar with their breasts and aware of any changes, which should be evaluated by their doctors.

The U.S. Preventive Services Task Force, whose guidelines have historically influenced Medicare coverage, made waves in 2009 when it recommended mammograms every other year starting at age 50, to age 74. In draft recommendations released earlier this year, the group said mammograms for women in their 40s should be an individual decision based on preferences and health history, and that more research is needed to determine potential benefits or harms for scans for women aged 75 and older.

That panel also questioned the value of breast exams by doctors, citing a lack of evidence for any benefit or harm. It will examine the cancer society’s evidence review in finalizing its update, said Dr. Kirsten Bibbins-Domingo, the task force’s vice chair and a professor at the University of California, San Francisco.

Most health plans are required to cover screening mammograms free of charge as part of preventive care mandated by the Affordable Care Act, and many insurers cover the screenings starting at age 40.

Several doctor groups still recommend mammograms starting at age 40, including those representing radiologists and gynecologists, but the American Cancer Society’s breast cancer guidelines are the most widely followed, said Dr. Kenneth Lin, a family medicine physician at Georgetown University School of Medicine.

Lin said he supports the new guidance but that it will make his job more challenging at first, trying to explain to patients the changes and differences with other groups. Lin said he — and probably many other doctors — will likely continue doing physical breast exams, out of habit and because they consider the exams to be an important part of doctor-patient interactions.