An estimated 321,910 women in the United States will be diagnosed with breast cancer in 2026, according to the Breast Cancer Research Foundation. The median age at diagnosis is 62. But the highest increases in the percentage of women diagnosed with breast cancer — 16% — have been reported in women under 50, according to the foundation, marking a 1.4% increase between 2012 and 2021. That’s the largest increase of all breast cancer diagnoses in women.

What’s especially concerning is that women under 50 are 40% more likely to die of the illness than women older than 40. In women ages 20 to 49, breast cancer is the leading cause of death among all cancers.

Breast cancer surgical oncologist Dr. Alycia So, who practices at Fox Chase Cancer Center and Temple Health-Chestnut Hill Hospital, has seen the increase firsthand. Speaking of younger patients diagnosed with breast cancer, she says, “It’s not uncommon that at least one clinic day has at least one. Honestly, I had one clinic day where I had three patients under the age of 50, all with new breast cancer diagnoses. So, the numbers reflect my clinical experience.”

The youngest patient diagnosed with breast cancer seen by So was 22.

Receiving a diagnosis of breast cancer at a younger age means coming to it at a different stage of life — starting a career, starting a family.

“Cancer feels unjust to start with,” says So. “It’s, why do we have this? Why do we have to go through this? But when I see a young woman with breast cancer, it is cosmically unfair. It is heartbreakingly unfair. For families, for women, for the patient, it is earth-shattering. It rocks your world.”

Until this relatively recent uptick in breast cancer diagnoses in younger women, So says, people typically linked breast cancer to aging. “The longer you live, the longer your cells have time to act up,” she says. “But increasingly, we’re not seeing that.”

The big question: Why is this happening? Why are younger women developing breast cancer?

For a time, So suggests, experts thought it might be a question of improved screening.

“We have so much innovation, so much technology, especially when it comes to radiologic screening,” she explains. “My favorite phrase is, screening saves lives — because it does. The earlier we catch a breast cancer, the easier it is to treat, the better someone’s outcomes are. But I don’t think improved screening fully explains the increasing incidence.”

There is also a link between genetics and breast cancer in younger women, she notes. Women who inherit mutated BRCA (breast cancer) genes have a higher risk. Women who inherit these genetic variants also often develop breast cancer at a younger age, according to the National Cancer Institute.

Still, So suggests, pathogenic gene mutations don’t fully explain the increased incidence in breast cancer in women under 50.

Other factors are suspected to be at work, everything from early obesity and unhealthy diet to delayed childbearing and even exposure to microplastics, she says.

Reproductive shifts in particular are suggestive. There’s a link between lifetime exposure to the hormone estrogen and development of breast and other cancers in women. The longer women delay childbearing — or if they elect not to have children at all — the longer they’re exposed to estrogen, says So. “When you have kids earlier, it affects your hormone levels. If you don’t have children, you don’t have that kind of fluctuation in your estrogen levels.”

Likewise, breastfeeding lowers estrogen levels, she adds. But if a women doesn’t breastfeed, her body is exposed to estrogen for longer periods.

Regardless of the cause — or causes — breast cancer in younger women is especially concerning because it tends to be more aggressive, So says. Making matters worse, because many of these women haven’t yet reached age 40 — when routine annual mammograms begin — their cancer is often more advanced by the time it is detected. “It’s always trickier to treat when it’s at a later stage,” she says.

For younger women, So suggests, early detection starts with a good comprehensive patient history by a primary care physician or ob-gyn.

“Obviously, you do an exam, but you want to know what are the risk factors that can contribute to somebody who might have a higher incidence of breast cancer,” she says. “If there’s a strong family history, should these women undergo genetic testing? Should they meet with a genetic counselor? Should they get high-risk screening?”

Screening for high risk-women consists of annual mammograms and MRIs beginning as young as age 30, according to American Cancer Society guidelines.

For women who fall into higher-risk categories, Fox Chase also has a robust risk assessment program called Breast Examination and Screening Team (B.E.S.T.).

“It’s for women who might have abnormal imaging, or they feel a lump, or they have a higher breast density,” So explains. “The team works with the surgeons, the medical oncologists, the risk assessment program. I’m really proud of it because I think it answers a lot of questions. It’s a resource for providers and for patients about patient risk profiles.”

So also advocates for lowering your risk in other ways.

“The American Cancer Society will say you want to decrease red meats, eat leafier greens, and you want to move your body and maintain and healthy weight,” she says. “All things in service of lowering inflammation and lowering your incidence not only of breast cancer, but just cancers in general and general health, of course.”

And don’t forget self-exams, she says.

“We used to tell women, every day, check yourself, and that unexpectedly led to a lot of anxiety. So, what we really say is you want to have an idea what’s going on with your body, specifically in relation to your breasts. So when you’re showering or putting on lotion or as you’re getting dressed, just notice differences and anything that feels different, bring it up with your doctor.”

More information from the National Cancer Institute: cancer.gov/about-cancer/causes-prevention/genetics/brca-fact-sheet#what-are-brca1-and-brca2

And from the American Cancer Society: cancer.org/cancer/types/breast-cancer/screening-tests-and-early-detection/american-cancer-society-recommendations-for-the-early-detection-of-breast-cancer.html