If women began breast cancer screening at 40 and received mammograms yearly, fewer would die from the disease than if they had started regular screening at a later age and screened less often, according to a new study in the journal Cancer.

But the research also found that with earlier screening, more women may experience false alarms—mammography results that look dangerous but aren’t. These can cause anxiety and lead to extra imaging tests or biopsies.

That’s one reason guidelines by major expert groups recommend less frequent screening later in life. 

The new study used a computer model to compare the effects of three strategies for breast cancer screening:
• Screening every year starting at age 40
• Screening every year starting at age 45, then switching to every other year from age 55 to 79
• Screening every other year from age 50 to 74

R. Edward Hendrick, Ph.D., a clinical professor of radiology at the University of Colorado School of Medicine and one of the study’s authors, says he isn’t surprised by the results.

“If you screen more frequently and over a wider range of ages, you’re going to detect more breast cancer," he says, "and in doing that, you’re going to save more lives.” 

But most medical groups don't agree that annual screening starting at age 40 is optimal. The American Cancer Society (ACS), for instance, recommends yearly screening from 45 to 54 and every other year from 55 on, and the U.S. Preventive Services Task Force (USPSTF) recommends screening every other year from age 50 to 74.

The groups do agree that by age 50, women should get regular mammography screening, with the option to start as early as 40. These recommendations are based on decades of research involving hundreds of thousands of real women, one reason the new study’s results—based on a computer model—don’t call those guidelines into question.

The potential negative effects of breast cancer screening—including false alarms and treatment of cancers that may never harm a woman during her life—mean that pinpointing the right age to start getting mammograms isn’t simple.

Here’s what you need to know about the results of this new study, and how to determine which strategy for breast cancer screening is right for you.

What Did the New Study Show?

The research team's statistical models used a number of variables, including breast cancer incidence, survival, and mortality trends, to estimate how many breast cancer deaths would be prevented if all women in the U.S. adhered to the various screening strategies.

They also estimated how many times women would be called back for additional testing after a suspicious mammography result, and how many women would undergo biopsies that turned out negative for breast cancer.

Overall, the picture these models paint is clear: The more often women receive mammograms, the more breast cancer deaths are averted—but the more unnecessary follow-up tests are undertaken as well.

It’s important to remember, says Marvin M. Lipman, M.D., Consumer Reports’ chief medical adviser, that computer models differ from the way things play out in the real world. The models used in this study, for example, assume that 100 percent of women adhere perfectly to the screening schedules, follow-up exams, and any breast cancer treatment.

“But in real life, everything is not optimal,” Lipman says. “People miss or skip their appointments, or they never return for additional testing. Without examining the behavior and disease patterns of actual people, it’s difficult to know how to apply these results to the real world.”

Does This Change Screening Guidelines?

Over the past few years, major medical groups seem to have grown closer to a consensus on when women should begin mammography screening to detect breast cancer, and how often they should be screened.

Earlier this year, for example, the American College of Obstetricians and Gynecologists (ACOG) updated its recommendations, bringing its guidelines closer to those of the American Cancer Society and the USPSTF.

Previously, ACOG recommended that all women start screening at 40. Now the organization advises all women to start screening no later than age 50, with the option of starting in their 40s if they prefer.

From Hendrick’s perspective, the fact that earlier, more frequent screening potentially saves more lives outweighs the risks of screening. In addition, he notes, breast tumors discovered in women in their 40s tend to be more fast-growing than cancers discovered later in life. “If you’re going to screen in your 40s, it really needs to be annually,” he says.

But other experts place more emphasis on the harm that breast cancer screening can cause. For example, having to go back for a biopsy that turns out to be negative may make some women less likely to get their next routine mammogram, according to a 2013 study conducted in the U.K.

And many scientists believe that at least some women may become cancer patients needlessly through overdiagnosis (diagnosing slow-growing cancers that would never cause harm) and overtreatment (undergoing cancer treatment unnecessarily).

Estimates vary widely on how often a woman is diagnosed with a cancer that would never have hurt her during her lifetime, but they range from 3 to 50 percent.

What Should You Do?

“Women should talk with their doctors and weigh their own personal risk factors when making choices about when to start receiving mammograms and how often to screen,” Lipman says.

Women at a high risk of breast cancer should come up with a customized screening strategy with their doctor. You're in this group if you’ve already had breast cancer, have a first-degree relative who had breast cancer, had genetic testing and know you carry a BRCA1 or BRCA2 genetic mutation, or have a history of multiple chest X-rays or radiation treatments to the chest​ (as with lymphoma therapy).

Even if you’re not in the above group, other—more minor—risk factors may factor in your decision. These include:
• A personal history of noncancerous breast conditions, including atypical hyperplasia or dense breasts
• Menstrual periods that started before age 12 or continued after 55, or not having a child before 30
• A history of hormone replacement therapy
• Obesity, smoking, or excessive alcohol consumption

According to Lipman, the more of these risk factors you have, the more aggressive you may want to be with screening, beginning in your 40s or screening yearly instead of every other year.

One key takeaway from this new research, according to Hal Lawrence, M.D., chief executive officer of ACOG, is that starting at 40, women should be talking with their doctor about the harms and benefits of screening. You may decide that waiting to start mammography is right for you, but you should have the option. “I think over the last 10 or 15 years we’ve confused the heck out of women” when it comes to breast cancer screening, he says.

Lipman adds: “Women need to be aware that they can tailor their screening for their wishes, desires, and anxieties”—as well as the absence or presence of risk factors.