Several young celebrities have announced breast cancer diagnoses in the past year — a public reminder that rates are rising among women under 50 in the U.S.

Pop singer Jessie J, 37, revealed this week on Instagram that she has early-stage breast cancer and plans to undergo surgery later this month. Katie Thurston, former star of “The Bachelorette,” has documented her journey with Stage 4 breast cancer on social media after being diagnosed earlier this year at age 34. And actor Danielle Fishel, known for her role on the ’90s sitcom “Boy Meets World,” revealed her diagnosis at age 43 to fans last summer.

New breast cancer diagnoses in young women have gone up considerably in the last decade. From 2012 to 2021 — the most recent decade of data — the rate increased 1.4% annually in women under 50, compared with 0.7% annually in women 50 and up.

The trend applies to all racial or ethnic groups, particularly Asian American and Pacific Islander women under 50, for whom diagnoses have risen nearly 50% since 2000. Black women have the highest rate of breast cancer before age 40 and are most likely to die of the disease.

Women under 40 generally aren’t advised to get mammograms unless they have a strong risk factor for breast cancer, such as a family history or genetic mutation. In that case, the American Cancer Society recommends mammograms starting at age 30, plus an annual breast MRI.

Several breast cancer doctors said younger patients and their clinicians should be careful not to dismiss symptoms such as a lump or nipple discharge.

“The thought was always, if you had a change in your breast but you were a young woman, it was probably nothing,” said Dr. Rani Bansal, an assistant professor at the Duke University School of Medicine. “As we’re seeing more and more younger women get diagnosed … we need to take these cases seriously.”

Dr. Oluwadamilola Fayanju, chief of breast surgery at Penn Medicine, said her youngest patient diagnosed with cancer was just 17. She recommended that young women with symptoms go to a center that’s experienced in breast imaging.

For women with an elevated risk of breast cancer, she said, “it may be better for you to be connected with a dedicated breast provider who can keep a close eye on you and do regular exams even well before 40.”

Breast cancer in young women is often more aggressive

As treatment options for breast cancer have improved, the overall mortality rate among younger women with the disease declined from 2010 to 2020, according to research presented in April at the American Association for Cancer Research meeting in Chicago.

But young women are still more likely to be diagnosed with aggressive breast cancer compared with older women. One reason could be that they’re not getting screened as much, so it’s harder to catch cases early. Young women are also more likely to be diagnosed with triple-negative breast cancer, which tends to spread fast and has fewer treatment options.

Dr. Virginia Borges, a professor of medical oncology at the University of Colorado Cancer Center, said all women diagnosed with breast cancer before age 35 have a higher likelihood of the cancer spreading to the rest of their bodies, for reasons doctors don’t fully understand.

“It’s like this great big puzzle of all these different factors that can contribute to why we see these cancers behave the way they do,” Borges said.

Bansal said doctors are hoping to learn more about which treatments are better suited to women under 50.

“We need more data to better tailor our treatments towards younger women,” she said. “A lot of the studies that are done are in older women.”

Lifestyle, environment, hormones

There are several mysteries as to why younger women are diagnosed with breast cancer at higher rates. Doctors generally agree that multiple factors are at play, including lifestyle, hormones and environmental exposures.

Diets high in ultra-processed foods or a lack of physical activity can lead to obesity, which in turn elevates one’s cancer risk. Women in their 30s and 40s have also increased their alcohol consumption in recent decades, and drinking alcohol is linked to breast cancer.

Exposure to environmental toxins such as air pollution, forever chemicals or microplastics could also play a role.

“By the time women now in their 40s were babies, every single baby bottle had BPA. Everyone had Teflon pans in their home. Everyone was spraying Scotchgard around their home,” said Suzanne Price, CEO of Breast Cancer Prevention Partners, a nonprofit that works to eliminate exposure to toxic chemicals.

Several researchers said more data is needed to definitively draw that link.

“Hopefully within the next few years, we should be having more insight into how those early life exposures drive the risk of breast cancer,” said Dr. Adetunji Toriola, a professor of surgery at Washington University School of Medicine.

Some studies have suggested that chemical hair straighteners, which are predominantly used by Black women, may be associated with an increased risk of breast cancer. Fayanju said the straighteners “can potentially have effects on our ability to process hormones in our body and how those hormones then interact with cells in our breasts, which have receptors for those hormones.”

Changes in women’s reproductive lives might have some effect as well.

Girls in the U.S. are starting their periods slightly earlier in life compared with decades ago. That may increase the length of time in which they’re exposed to higher levels of estrogen — a hormone that in some cases can feed cancer cells. A study last year found an increase in the number of women ages 20 to 49 diagnosed with breast cancer that was responsive to estrogen.

Many women are also delaying childbirth until their 30s and 40s, which increases the risk of postpartum breast cancer — cancers that occur within five to 10 years of giving birth that appear to be linked to changes in the breasts during that time. Borges estimated that there are about 18,000 new cases of postpartum breast cancer each year.

“How do you get from the millions of women who are having children without ending up with one of these breast cancers to the 18,000 or so who are going to get one of these breast cancers?” she said. “We’re still working on figuring that out. Age is important.”