At age 45. At age 50. Once a year. Once every two years. If you’re trying to find some consensus on when and how often women should get screened for breast cancer with mammograms, you’re not going to find universal agreement.

As confusing and conflicting as the advice seems, the back and forth is leading to a more personalized vision for breast cancer screening that should be more effective for individual women. Rather than blanket statements that can sweep women who are at high risk into the same screening schedule as lower risk women, for example, what’s emerging is a more flexible scheme, after discussion with a woman’s doctor, that’s better tailored to her own risk. The major groups agree that most women should get their first screening between age 45 and 50, and that women with a family history of breast cancer should start earlier.

When mammograms emerged in the 1970s as a tool for detecting breast cancer, it was in the era of the War on Cancer. The early understanding of cancer then—as it still is now—was that finding cancer early, when tumors are still small, leads to the best chance for treating it and helping women live longer. Cancer was stigmatized tremendously at the time and often taboo to talk about, so doctors struggled to get the public to accept the disease and introduce the idea of getting tested as early as possible to control it. The general advice was to screen as early as needed—and as often as practically possible.

However, while the assumption was that screening would lead to longer lives and more lives saved from cancer, there was conflicting data on whether that was actually the case. It made intuitive sense, but studies showed that women who were screened regularly did not necessarily avoid dying from breast cancer, compared to women who weren’t screened. Even the age at which doctors suggested that women start getting mammograms—40—was somewhat arbitrary, based on the fact that cancer, and breast cancer in particular, is generally a disease that occurs in older people.

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Those studies also show that the high number of lesions found by mammography, some of which were false positives, led to a surge in additional testing, biopsies and radical treatments, including mastectomy and even prophylactic mastectomy, in which women decide to remove both breasts even if there are tumors in only one. Studies have found that mammograms in the U.S. can lead to 30% higher false positive results in which the suspicious lesion turns out not to cancerous.

In recent decades, new scientific evidence started trickling in, since enough women had been screened and followed for years to document their rates of breast cancer and causes of death. When the potential benefit of saving lives from breast cancer were weighed against the risks of over diagnosis and over treatment, those results were an eye-opener: they failed to consistently show that mammograms did more good than harm.

Still, the intuitive message behind screening—that looking for cancer helps to find it and treat it—was so strong that it was hard for doctors and the public to question mammograms. Then the U.S. Preventive Services Task Force (USPSTF), a group of independent experts convened by the government that is charged with taking on such topics, reviewed the evidence in 2009. The task force’s mission is to provide advice based on what the most rigorous scientific studies show. If there is no proof that something is beneficial, then the task force does not recommend it.

When the USPSTF reviewed the literature on mammograms, they came to a surprising conclusion. There was little evidence to support the benefits of mammograms in younger women, they found. For them, the risks of having biopsies of suspicious lesions, or additional procedures if the screening picked up lesions, outweighed the benefits in protecting them from advanced breast cancer. After analyzing the data, they recommended that most women start mammogram screening at age 50, not 40, and that they get screened once every two years rather than annually.

The recommendation wreaked havoc in the cancer community and caused confusion among women. Breast cancer advocates were convinced that the advice would lead to a rise in breast cancer rates, not to mention deaths from breast cancer, if more women delayed screening and had their cancers picked up later, when treatments are less effective. The Susan G. Komen group, for example, voiced concerns that younger women may feel less urgency to get mammograms. “There is enough uncertainty about the age at which mammography should begin and the frequency of screening that we would not want to see a change in policy for screening mammography at this time,” the group said in a 2009 statement responding to the USPSTF findings.

Within a few years, studies did show that anywhere from 6% to 17% fewer women in their 40s, depending on their ethnicity, were getting mammograms. According to the latest figures from the SEER cancer database, however, deaths from breast cancer continued to decline at the same rate, by nearly 2% each year from 2005-2014, even after the USPSTF recommendations.

The rate of new breast cancer diagnoses also has not shot up as a result of the advice. Researchers say that may reflect the fact that some of the cases detected by mammograms among women in their 40s may not have been cancer after all, but lesions that were picked up by the test and then removed or treated.

The American Cancer Society now takes the middle road between the previous guidelines and the USPSTF in its advice, saying that women should start talking to their doctors about mammogram screening when they reach 45. Most groups do agree that women after age 75 should not be screened regularly—only if they have a family history or other reason to suspect they are at high risk for developing cancer.

The lesson from the shifting advice is that it’s still important for women to get screened for breast cancer, since detecting tumors early is linked to longer lives and fewer deaths from the disease. But when women should start getting the tests, and how often, depends a lot on her particular set of risk factors for breast cancer: whether she smokes and whether she has a family history of the disease, for example. The latest recommendations reflect a refining of mammogram advice toward a more personalized regimen that women and their doctors come up with that will, in the end, give women their strongest chance of preventing and surviving the disease.

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