Why are more young women being diagnosed with breast cancer?

Siobhan Donovan was a runner who ate her vegetables, didn’t smoke and drank alcohol only socially. She had no family history of cancer. So when she experienced swelling in her breast at the end of what she called “a textbook pregnancy” with her third child, she and her doctors expected nothing serious.

They were wrong. Mrs Donovan, who was 33 at the time, had metastatic breast cancer that had spread to her bones.

“I was really just in shock,” she said, adding that at the time she didn’t even know the meaning of the word “metastasis.”

Like Mrs Donovan, a growing number of younger women are being diagnosed with breast cancer, according to new estimates released on Thursday by American Cancer Society. Between 2012 and 2021, the incidence of breast cancer overall increased by about 1 percent each year, while the incidence among women under 50 increased by about 1.4 percent each year. Patients with “young-onset breast cancer” — which clinicians typically define as diagnosed before the age of 40 — are more likely than older patients to have aggressive forms of the disease, Dr. Ann Partridge, interim chair of medical oncology at the Dana-Farber Cancer Institute in Boston.

The rise in breast cancer rates among younger patients occurs as the incidence of other early-onset cancers — including colon, stomach, kidney and liver cancers — is also increasing, although cancer among patients under 50 is still relatively rare overall, Dr. Partridge said.

In 2024, there were just under 51,000 new cases of invasive breast cancer among women under 50, compared with about 260,000 cases among women 50 and older, according to Statistics from the American Cancer Society.

“Cancer is a disease of aging,” said Dr. Partridge. “But if you’re going to get cancer in your 30s or 40s, if you’re a woman, it’s most likely to be breast cancer.”

Researchers aren’t sure what’s driving the rise in early-onset breast cancer. While mammography screening has led to an overall increase in breast cancer diagnoses and a decrease in breast cancer deaths, it is not likely what is driving the increase in diagnoses among younger women, experts said. The reason is that prevention guidelines have not historically recommended routine mammography until women were at least in their 40s, so cancers that occurred earlier would not have been detected through screening. (Last year, United States Preventive Services Task Force lowered the recommended age to begin screening at 40, from 50, in part because more women in their 40s are being diagnosed.)

Rather, doctors and epidemiologists believe the increase is driven by a combination of shifts in women’s fertility and environmental or lifestyle factors.

Dr. Adetunji Toriola, professor of surgery at Washington University School of Medicine in St. Louis, studied incidence of breast cancer in women under 50 years of age by breed, stage and type. He found that the increase over the past 20 years is largely driven by breast cancers fueled by the sex hormone estrogen – like the type Ms Donovan was diagnosed with.

Breast cells are equipped with estrogen receptors, and estrogen helps drive cell growth and proliferation. “It’s really like fertilizer,” said Dr. Graham A. Colditz, an epidemiologist and associate director for prevention and control at the Alvin J. Siteman Cancer Center at Washington University. But too much estrogen over a lifetime can lead to the formation and growth of cancer cells.

Over time, there have been several population-level shifts that affect women’s lifetime exposure to estrogen, potentially contributing to increased breast cancer rates across all ages, experts said.

For example, the average age at which a girl gets her first period has decreased over the past 60 years. At the same time, women are entering menopause later in life, said Dr. Toriola. Both of these changes increase women’s lifetime exposure to estrogen.

Pregnancy can have different effects at different stages of life. In the short term, pregnancy can increase the risk of breast cancer. But getting pregnant, especially before age 30, and breastfeeding both have a long-term protective effect against breast cancer later in life — an effect thought to be partly due to how these conditions change breast cells, said Dr. Partridge. As more women delay pregnancy or choose not to have children at all, they may lose that protection.

Alcohol use, especially during adolescence and early adulthood, also clearly increases the risk of breast cancer. Alcohol can damage DNA and is linked to increased exposure to estrogen, experts said. This is particularly concerning given that women are now drinking — and drinking heavily — at the same rate as men, said Dr. Colditz.

Changing dietary habits and reduced physical activity can also increase the risk of breast cancer. Several studies, including Dr. Colditz’s, has suggested that women’s height and growth rate are associated with increased risk of breast cancer, signaling the impact of childhood nutrition.

Research has also shown that exercise reduces the risk of breast cancerindependent of its effect on weight or body mass index, said Dr. Colditz.

Obesity itself appears to have mixed effects, increasing the risk of breast cancer among postmenopausal women but decreasing it for premenopausal women. But weight gain — independent of body mass index — has been linked in several studies with increased risk of breast cancer in women under 50.

Although the majority of breast cancers in younger women, as in older women, are of the type related to sex hormones, younger women are more likely to be diagnosed with more difficult to treat cancers unrelated to estrogen, such as “triple-negative” breast cancer, said Dr. Mariya Rozenblit, an assistant professor of medical oncology at the Yale School of Medicine. They are also more likely than older women to have potentially cancer-causing forms of genes such as BRCA1 or BRCA2, although such genetic predispositions still only account for a fraction of cancers in this age group.

Younger women also aren’t routinely screened, so by the time they have symptoms and are diagnosed, any tumor is likely to be larger and spread more quickly, Dr. Rose blitz.

Because of the greater chances of being diagnosed with more aggressive disease, younger women have lower survival rates on average than older women, said Dr. Partridge – although overall breast cancer survival continues to improve with newer treatments and fewer delays in diagnosis.

“There is a difference there, but the vast majority of young women will be short- and long-term survivors,” said Dr. Partridge.

It has been almost three years since Mrs Donovan was first diagnosed. She went back to work as an elementary school teacher in Worcester, Massachusetts, and ran the Boston Marathon in 2024. But the chemotherapy and treatments she initially underwent were not enough to stop her cancer from progressing, and she had to start a new drug that makes her feel tired and constantly sick. Her youngest son has never seen her with a full head of hair. She recently had her first difficult conversation about breast cancer with her 5-year-old daughter.

“Initially, I never thought I’d see three years,” she said. “And even though it’s hard, I’m still going.”