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NEW YORK - For many women, getting a mammogram is already one of life's more stressful experiences.

Now, women in their 40s have the added anxiety of trying to figure out if they should even be getting one at all.

A government task force said Monday that most women don't need mammograms in their 40s and should get one every two years starting at 50 — a stunning reversal and a break with the American Cancer Society's long-standing position. What's more, the panel said breast self-exams do no good, and women shouldn't be taught to do them.

The news seemed destined to leave many deeply confused about whose advice to follow.

"I've never had a scare, but isn't it better to be safe than sorry?" asked Beth Rosenthal, 41, sitting in a San Francisco cafe on Monday afternoon with her friend and their small children. "I've heard of a lot of women in their 40s, and even 30s, who've gotten breast cancer. It just doesn't seem right to wait until 50."

Her friend agreed. "I don't think I'll wait," said Leslie David-Jones, also 41, shaking her head.

For most of the past two decades, the American Cancer Society has been recommending annual mammograms beginning at 40, and it reiterated that position on Monday. "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.

But the government panel of doctors and scientists concluded that getting screened for breast cancer so early and so often is harmful, causing too many false alarms and unneeded biopsies without substantially improving women's odds of surviving the disease.

"The benefits are less and the harms are greater when screening starts in the 40s," said Dr. Diana Petitti, vice chair 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.

Experts expect the 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 says:

• 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.