(CNN) In a move sure to befuddle women -- and anger some breast cancer survivors -- the American Cancer Society has issued new guidelines saying less screening for breast cancer is better than more.

The venerated cancer organization says women should start getting mammograms at 45 instead of 40, and that everyone can skip the routine manual breast checks by doctors.

An exhaustive review of the medical literature shows these measures just aren't very effective, according to the group. "The chance that you're going to find a cancer and save a life is actually very small," said Dr. Otis Brawley, the society's chief medical officer.

Now three key groups -- the American College of Obstetricians and Gynecologists , the American Cancer Society, and the U.S. Preventive Services Task Force -- recommend different ages for starting regular mammograms: 40, 45 and 50 respectively.

While mammograms save lives, they can also cause harm, and each group does a different job of balancing the pros and cons.

Earlier testing is not necessarily better

The problem with mammograms is that they have a relatively high false positive rate, which means women sometimes have to undergo painful and time-consuming tests only to find out they never had cancer in the first place.

The chances of false positives are especially high for women under 45, as they have denser breasts and tumors are harder to spot on an image. "If she starts screening at age 40, she increases the risk that she'll need a breast cancer biopsy that turns out with the doctor saying 'You don't have cancer, so sorry we put you through all this,'" Brawley said.

He said he knows women who've had false positives year after year. "False positives are a huge deal," he said. "These women are so frightened and inconvenienced they swear off mammography for the rest of their lives."

Six years ago, the federal government's Preventive Services Task Force caused a furor when it declared that women in their 40s didn't need to get routine mammograms. Younger women whose breast cancers were caught by mammograms angrily responded that they would have been dead if they'd followed that guideline.

They said they'd gladly risk a false positive, with all the inconvenient and sometimes painful followup, for the chance of finding a cancer.

Learning from that experience, the American Cancer Society has sought to soften its message, emphasizing that women in their early 40s should still be able to get mammograms if they want them, as long as they understand the risks.

There's the risk of a false positive, plus the risk that a mammogram could catch a very small breast cancer that will go way on its own, or never progress to the point that it hurts a woman. In other words, a mammogram could catch a tumor that isn't really worth catching.

Several reports discuss the spontaneous regression of cancer, including breast cancer

But since doctors can't reliably discern the harmful from the harmless cancers, they treat them all. This means some women are getting potentially harmful treatments, such as radiation, chemotherapy and surgery, when their tumor would never have caused a problem, Brawley says.

A Canadian study looked at 44,925 women who were screened for breast cancer, and 106 of them fell into this category and were treated for breast cancer "unnecessarily," according to a review in the New England Journal of Medicine.

New guidelines have their critics

While agreeing with the American Cancer Society that mammograms aren't perfect, some advocates for women criticized the group's new guidelines. First, they said the society looked mostly at studies of film mammography, which in the United States has almost been entirely replaced by digital mammography.

Digital mammograms generate clearer images and do a better job of finding cancer and have a lower false positive rate.

"It's like standard versus HD TV," said Dr. Therese Bevers, the chair of the National Comprehensive Cancer Network's guidelines panel for breast cancer screening and diagnosis, and the medical director of the Cancer Prevention Center at the MD Anderson Cancer Center.

Second, critics said the cancer society looked only at whether screening saved a woman's life, and not at whether screening caught a cancer early, so the woman could avoid the most drastic treatments, such as chemotherapy or mastectomy.

"The American Cancer Society made the value judgment that screening is only worth it if improves survival," said Dr. Marisa Weiss, a breast cancer survivor and president of Breastcancer.org. "There's an arrogance to that. Let women decide what's meaningful to them."

Insurance companies also decide

The new guidelines also state that women over age 55 can choose to get a mammogram every other year, since breast cancers in post-menopausal women tend to develop more slowly.

To a great extent it will be insurance companies that decide at what age women get mammograms. In 2009, they typically continued to pay for mammograms starting at age 40 even though the government's task for force recommended mammograms starting at age 50.

But it's not clear what they'll do now that the American Cancer Society has also raised the age for mammograms.

"(Insurance) plans will certainly take these updated recommendations into account when evaluating their coverage policies," Clare Krusing, a spokeswoman for America's Health Insurance Plans, wrote to CNN in an email.

The new guidelines are meant for women at average risk of breast cancer. The society says women with a family history or who carry a gene that predisposes them to breast cancer may need to start screening earlier and more frequently.

As for the recommendation to discontinue routine manual breast exams by doctors, many advocates for women with breast cancer agree there's a lack of good evidence that they save lives, but some said they saw no reason to get rid of them.

"It's a free and added way of knowing whether or not a lump is there," said Leigh Hurst, founder of the Feel Your Boobies Foundation.

In the end, with so many different opinions on preventing breast cancer, experts are worried women will throw up their hands.

"Our biggest concern is that this will create a lot of potential havoc in the day-to-day practice of caring for women," said Dr. Christopher Zahn, the vice president of practice activities for ACOG.