NEW YORK — Most women don’t need a mammogram in their 40s and should get one every two years starting at 50, a government task force said Monday. It’s a major reversal that conflicts with the American Cancer Society’s long-standing position.

Also, the task force said breast self-exams do no good and women shouldn’t be taught to do them.

For most of the past two decades, the cancer society has been recommending annual mammograms beginning at 40.

But the government panel of doctors and scientists concluded that getting screened for breast cancer so early and so often leads to too many false alarms and unneeded biopsies without substantially improving women’s odds of survival.

“The benefits are less and the harms are greater when screening starts in the 40s,” said Dr. Diana Petitti, vice chairwoman of the panel.

The new guidelines were issued by the U.S. Preventive Services Task Force, whose stance influences coverage of screening tests by Medicare and many insurance companies.

But Susan Pisano, a spokeswoman for America’s Health Insurance Plans, an industry group, said insurance coverage isn’t likely to change because of the new guidelines. No changes are planned in Medicare coverage either, said Dori Salcido, spokeswoman for the Health and Human Services department.

Experts expect the task force revisions to be hotly debated, and to cause confusion for women and their doctors.

“Our concern is that as a result of that confusion, women may elect not to get screened at all. And that, to me, would be a serious problem,” said Dr. Len Lichtenfeld, the cancer society’s deputy chief medical officer.

The guidelines are for the general population, not those at high risk of breast cancer because of family history or gene mutations that would justify having mammograms sooner or more often.

The new advice states:

• Most women in their 40s should not routinely get mammograms.

• Women 50 to 74 should get a mammogram every other year until they turn 75, after which the risks and benefits are unknown. (The task force’s previous guidelines had no upper limit and called for exams every year or two.)

• The value of breast exams by doctors is unknown. And breast self-exams are of no value.

Medical groups such as the cancer society have been backing off promoting breast self-exams in recent years because of scant evidence of their effectiveness. Decades ago, the practice was so heavily promoted organizations distributed cards that could be hung in the shower demonstrating the circular motion women should use to feel for lumps in their breasts.

The guidelines and research supporting them were released Monday and are being published in today’s issue of the Annals of Internal Medicine.

The new advice was sharply challenged by the cancer society.

“This is one screening test I recommend unequivocally, and would recommend to any woman 40 and over,” the society’s chief medical officer, Dr. Otis Brawley, said in a statement.

The task force advice is based on its conclusion that screening 1,300 women in their 50s to save one life is worth it, but screening 1,900 women in their 40s to save a life is not, Brawley wrote.

That stance “is essentially telling women that mammography at age 40 to 49 saves lives, just not enough of them,” he said. The cancer society feels the benefits outweigh the harms.

International guidelines call for screening to start at age 50; the World Health Organization recommends the test every two years.

Breast cancer is the most common cancer and the second leading cause of cancer deaths in American women. More than 192,000 new cases and 40,000 deaths from the disease are expected in the U.S. this year.

Mammograms can find cancer early, and two-thirds of women over 40 report having had the test in the previous two years. But how much they cut the risk of dying of the disease, and at what cost in terms of unneeded biopsies, expense and worry, have been debated.

In most women, tumors are slow-growing, and that likelihood increases with age. So there is little risk by extending the time between mammograms, some researchers say. Even for the minority of women with aggressive, fast-growing tumors, annual screening will make little difference in survival odds.