Well, it looks like good news, but you know the health insurance coverage is going to change. Not so good for women who are high-risk.

Most women should start regular breast cancer screening at age 50, not 40, according to new guidelines released Monday by an influential group that provides guidance to doctors, insurance companies and policy makers.

The new recommendations reverse longstanding guidelines and are aimed at reducing harm from overtreatment, the group says. It also says women age 50 to 74 should have mammograms less frequently — every two years, rather than every year. And it said doctors should stop teaching women to examine their breasts on a regular basis.

Just seven years ago, the same group, the United States Preventive Services Task Force, with different members, recommended that women have mammograms every one to two years starting at age 40. It found too little evidence to take a stand on breast self-examinations.

The group’s new guidelines, which are different from those of some professional and advocacy organizations, are published online in The Annals of Internal Medicine They are likely to touch off yet another round of controversy over the benefits of screening for breast cancer.

Dr. Diana Petitti, vice chairwoman of the task force and a professor of biomedical informatics at Arizona State University, said the guidelines were based on new data and analyses and were aimed at reducing the potential harm from overscreening.

While many women do not think a screening test can be harmful, medical experts say the risks are real. They include unnecessary tests, like biopsies that can create extreme anxiety. And mammograms can find cancers that are better off not found. Some cancers grow so slowly that they never would be noticed in a woman’s lifetime. When they are found, women end up being treated unnecessarily.

Over all, the report says, the modest benefit of mammograms — reducing the breast cancer death rate by 15 percent — must be weighed against the harms. And those harms loom larger for women in their 40s, who are 60 percent more likely to experience them than women 50 and older but are less likely to have breast cancer, skewing the risk-benefit equation. The task force concluded that one cancer death is prevented for every 1,904 women age 40 to 49 who are screened for 10 years, compared with one death for every 1,339 women age 50 to 74, and one death for every 377 women age 60 to 69.

The guidelines are not meant for women at increased risk for breast cancer because they have a gene mutation that makes the cancer more likely or because they had extensive chest radiation. The task force said there was not enough information to know whether those women would be helped by more frequent mammograms or by having the test in their 40s. Other experts said women with close relatives with breast cancer were also at high risk.

Dr. Petitti said she knew the new guidelines would be a shock for many women, but, she said, “we have to say what we see based on the science and the data.”

But the new report conflicts with advice from groups like the American Cancer Society and the American College of Radiology. They are staying with their guidelines advising annual mammograms starting at age 40.

The cancer society, in a statement by Dr. Otis W. Brawley, its chief medical officer, agreed that mammography had risks as well as benefits but, he said, the society’s experts had looked at “virtually all” the task force and additional data and concluded that the benefits of annual mammograms starting at age 40 outweighed the risks.

Other advocacy groups, like the National Breast Cancer Coalition, Breast Cancer Action, and the National Women’s Health Network, welcomed the new guidelines.

“This is our opportunity to look beyond emotions,” said Fran Visco, president of the National Breast Cancer Coalition. The task force “is an independent body of experts that took an objective look at the data,” Ms. Visco said. “These are the people we should be listening to when it comes to public health messages.”

Some women, though, were not pleased.

“I know so many people who had breast cancer and survived, and what saved their lives was early detection,” Janet Doughty, 44, of San Clemente, Calif., said in a telephone interview. She said she had had an annual mammograms since her late 30s and would not stop now.

The guidelines are not expected to have an immediate effect on insurance coverage but should make health plans less likely to aggressively prompt women in their 40s to have mammograms and older women to have the test annually.

Congress requires Medicare to pay for annual mammograms. Medicare can change its rules to pay for less frequent tests if federal officials direct it to.

Private insurers are required by law in every state except Utah to pay for mammograms for women in their 40s.

But the new guidelines are expected to alter the grading system for health plans, which are used as a marketing tool. Grades are issued by the National Committee for Quality Assurance, a private nonprofit organization, and one measure is the percentage of patients getting mammograms every one to two years starting at age 40.

That will change, said Margaret E. O’Kane, the group’s president, who said it would start grading plans on the number of women over 50 getting mammograms every two years.

The message for most women, said Dr. Karla Kerlikowske, a professor in the department of medicine, epidemiology and biostatistics at the University of California, San Francisco, is to forgo routine mammograms if they are in their 40s.

Starting at age 50, Dr. Kerlikowske said, “the message is to get 10 mammograms in a lifetime, one every two years.” That way they get the most benefit and the least harm from the test. If women are healthy, she added, they might consider having mammograms every two years until age 74.

Nearly two-thirds of all women in their 40s had mammograms within the last two years, as did 72 percent of women age 50 to 65, according to an editorial by Dr. Kerlikowske that accompanies the report.

In order to formulate its guidelines, the task force used new data from mammography studies in England and Sweden and also commissioned six groups to make statistical models to analyze the aggregate data. In Europe, women are generally screened every two years starting at age 50.

The models were the only way to answer questions like how much extra benefit do women get if they are screened every year, said Donald A. Berry, a statistician at the University of Texas M.D. Anderson Cancer Center and head of one of the modeling groups.

“We said, essentially with one voice, very little,” Dr. Berry said. “So little as to make the harms of additional screening come screaming to the top.”

The harms are nearly cut in half when women have mammograms every other year instead of every year. But the benefits are almost unchanged.

The last time the task force issued guidelines for mammograms, in 2002, the reportwas announced by Tommy G. Thompson, then the secretary of health and human services. When the group recommended mammograms for women in their 40s, some charged the report was politically motivated. But Dr. Alfred Berg of the University of Washington, who was the task force chairman at the time, said “there was absolutely zero political influence on what the task force did.”

It was still a tough call to make, Dr. Berg said, adding that “we pointed out that the benefit will be quite small.”

In fact, he added, even though mammograms are of greater benefit to older women, they still prevent only a small fraction of breast cancer deaths.

Different women will weigh the harms and benefits differently, Dr. Berg noted, but added that even for women 50 and older, “it would be perfectly rational for a woman to decide she didn’t want to do it.”

Researchers worry the new report will be interpreted as a political effort by the Obama administration to save money on health care costs.

Of course, Dr. Berry noted, if the new guidelines are followed, billions of dollars will be saved.

“But the money was buying something of net negative value,” he said. “This decision is a no-brainer. The economy benefits, but women are the major beneficiaries.”