The risk keeps increasing slowly with age, but by 55, when most women have passed through menopause, tumors are less likely to be fast-growing or aggressive, and breast tissue changes in ways that make mammograms easier to read — so screening every other year is considered enough.

As for the decision to stop recommending clinical breast exams, the society said that there was no evidence that the exams save lives, but that there was evidence that they could cause false positives — meaning they could mistakenly suggest problems where none existed and lead to more tests. The exams can take five or six minutes that could be put to better use during office visits, said Dr. Kevin C. Oeffinger, the chairman of the cancer society subgroup that developed the guidelines and director of cancer survivorship at Memorial Sloan Kettering Cancer Center in New York.

According to the evidence review accompanying the guidelines, the benefit of regular mammography is that it can reduce the risk of dying from breast cancer by about 20 percent. Because breast cancer is less common in younger women, their baseline risk of dying is lower, and screening them saves fewer lives.

While younger women have less to gain from mammograms, the cancer society found, they incur all the potential harms. One harm is false positives, which can lead to more tests, including biopsies. A 2011 study cited in the article explaining the new guidelines found that 61 percent of women who had yearly mammograms starting at age 40 had at least one false positive by the time they were 50. Being tested every other year instead of every year can cut the false positive rate significantly, the JAMA Oncology article explaining the guidelines said, to about 42 percent from 61.

Some women consider false positives a small price to pay for the chance of identifying a cancer early. Others find being called back for more tests too nerve-racking.

Another potential risk of mammography is overdiagnosis, meaning some of the tiny cancers it finds might never progress or threaten the patient’s life. But because there is now no way to be sure which will turn dangerous, they are treated anyway.

There are no widely accepted figures on how often overdiagnosis occurs. Researchers think that it is mostly likely in women found to have ductal carcinoma in situ, or D.C.I.S., tiny growths in the milk ducts that may or may not evolve into invasive cancer. About 60,000 cases of D.C.I.S. are diagnosed in the United States each year.