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 some swelling in her breast near the end of what she called “a textbook-easy pregnancy” with her third child, she and her doctors expected nothing serious.
They were wrong. Ms. 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 back then she didn’t even know the meaning of the word “metastasis.”
Like Ms. Donovan, a growing number of younger women are being diagnosed with breast cancer, according to new estimates released Thursday by the American Cancer Society. Between 2012 and 2021, the incidence rate of breast cancer overall increased by about 1 percent each year, while the incidence rate among women under age 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, said Dr. Ann Partridge, interim chair of medical oncology at the Dana-Farber Cancer Institute in Boston.
The increase in breast cancer rates among younger patients is occurring as the incidence of other early-onset cancers — including colorectal, gastric, kidney and liver cancer — is also on the rise, though cancer among patients under 50 is still relatively rare overall, Dr. Partridge said.
In 2024, there were just under 51,000 new invasive breast cancer cases among women under 50, compared with about 260,000 cases among women 50 years of age and older, according to American Cancer Society statistics.
“Cancer is a disease of aging,” Dr. Partridge said. “But if you’re going to get a cancer in your 30s or 40s, if you’re a woman, it’s most likely to be breast cancer.”
Researchers are not certain what is driving the increase 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’s not likely what’s driving the increase in diagnoses among younger women, experts said. The reason is that prevention guidelines haven’t historically recommended routine mammograms until women were at least into their 40s, so cancers occurring earlier would not have been detected through screening. (Last year, the United States Preventive Services Task Force lowered the recommended age to begin screening to 40, from 50, in part because more women in their 40s are being diagnosed.)
Rather, physicians and epidemiologists believe the increase is being driven by a combination of shifts in women’s fertility and environmental or lifestyle factors.
Dr. Adetunji Toriola, a professor of surgery at Washington University School of Medicine in St. Louis, studied breast cancer incidence in women under 50 by race, stage and type. He found that the increase over the last 20 years has been driven largely by breast cancers fueled by the sex hormone estrogen — like the type with which Ms. Donovan was diagnosed.
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 cancerous cells.
Over time, there have been several population-level shifts that affect women’s lifetime exposure to estrogen, potentially contributing to increasing rates of breast cancer across all ages, experts said.
For example, the average age at which a girl gets her first period has dropped over the last 60 years. At the same time, women are entering menopause later in life, Dr. Toriola said. 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 may increase the risk of breast cancer. But becoming pregnant, especially before age 30, and breastfeeding both have a long-term protective effect against breast cancer later in life — an effect thought to result in part from how these conditions change breast cells, Dr. Partridge said. As more women delay pregnancy or choose not to have children at all, they may be losing that protection.
Alcohol use, especially in adolescence and early adulthood, also clearly increases breast cancer risk. Alcohol can damage DNA and is associated with increased exposure to estrogen, experts said. This is of particular concern given that women are now drinking — and drinking heavily — at similar rates to men, Dr. Colditz said.
Changing diets and reduced physical activity could also be increasing the risk for breast cancer. Several studies, including Dr. Colditz’s, have suggested that women’s height and growth rate are associated with increased breast cancer risk, signaling the impact of childhood nutrition.
Research has also shown that exercise reduces breast cancer risk, independent of its effect on weight or body mass index, Dr. Colditz said.
Obesity itself appears to have mixed effects, increasing breast cancer risk among postmenopausal women but decreasing it for premenopausal women. But weight gain — independent of body mass index — has been associated in multiple 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 harder-to-treat cancers not related to estrogen, like “triple-negative” breast cancer, said Dr. Mariya Rozenblit, an assistant professor of medical oncology at Yale School of Medicine. They are also more likely than older women to have potentially cancer-causing forms of genes like BRCA1 or BRCA2, though such genetic predispositions still only account for a fraction of cancers in this age group.
Younger women also are not routinely screened, so by the time they have any symptoms and receive a diagnosis, any tumor is likely to be larger and to be spreading more quickly, Dr. Rozenblit said.
Because of the greater odds of being diagnosed with more aggressive disease, younger women on average have lower survival rates than older women do, Dr. Partridge said — though overall breast cancer survival continues to improve with newer therapies and fewer delays in diagnosis.
“There’s a disparity there, but the vast majority of young women will be short- and long-term survivors,” Dr. Partridge said.
It has been nearly three years since Ms. Donovan was first diagnosed. She went back to work as a public-school teacher in Worcester, Mass., and ran the Boston Marathon in 2024. But the chemotherapy and treatments she initially underwent weren’t enough to stop her cancer from progressing, and she had to start a new drug that leaves her feeling fatigued 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 would have thought I would have seen three years,” she said. “And even though it’s hard, I’m still going.”
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