More younger women are getting breast cancer, and doctors are scrambling to understand why.

Rates of breast cancer in women under 50 have increased more than 15% in the last two decades, a rise that is almost entirely driven by an increase in estrogen receptor-positive breast cancer (ER+), a tumor subtype that needs the hormone estrogen to grow and spread.

Estrogen receptor-positive breast cancer is the most common type among all age groups. Because this specific type of breast cancer is almost exclusively causing the spike in younger women, some doctors suspect the trend is related to more people in the U.S. getting their periods earlier and having their first baby later.

“Women are being exposed to more estrogen over their lifespans,” said Dr. Alexandra Thomas, a medical oncologist at Duke Health. “That’s probably a big piece of it, but we don’t know why we’re seeing earlier periods.”

Numerous factors are likely behind the increase — including obesity, alcohol consumption, genetics and some hormonal birth control — but the role early puberty plays in a woman’s breast cancer risk is gaining attention. The age of puberty for girls has been falling for decades, especially among Black and Asian Americans.

“This work is still in the early stage,” said Dr. Adetunji Toriola, a molecular epidemiologist at Washington University’s Siteman Cancer Center. “But we do know from some of our previous work and the work of others that we know certain factors that may be responsible. We know that these reproductive factors can relate to breast cancers.”

Earlier menstruation, later motherhood

It’s long been known that starting puberty significantly early is linked to greater risk of developing breast cancer, although there’s little research yet about its influence on age at diagnosis.

A study published earlier this year in JAMA Network Open found that women born from 1950 to 1969 were, on average, 12.5 when they had their first period. Less than 9% had menarche — the scientific term for a girl’s first menstrual period — before age 11, which is considered early. Just 0.6% of these women had their first period before age 9, or very early.

Girls born from 2000 to 2005 got their first period, on average, just before they turned 12, half a year earlier than those born 40 to 50 years prior. Rates of early and very early menarche also increased, jumping to 15.5% and 1.4%, respectively.

“If you begin your period before 11, you’re at higher risk for breast cancer, and now the average age [of menarche] just keeps decreasing,” said Dr. Eleonora Teplinsky, head of breast and gynecologic medical oncology at Valley Health System in New Jersey.

Studies have suggested that for every year younger a girl is when she gets her period, her lifetime breast cancer risk increases by about 5%. Developing breasts early also increases breast cancer risk. A study of nearly 50,000 women found that girls who developed breasts before age 10 had a 23% higher risk of developing breast cancer in their lives compared to those who began to develop breasts at age 12 or 13.

When a woman starts menstruating, her ovaries release estrogen and progesterone. Both can play a role in increasing a woman’s risk for hormone-sensitive subtypes of breast cancer, which usually grow more slowly and are easier to treat. About 75% of breast cancers are sensitive to at least one hormone, usually progesterone or estrogen, according to the American Cancer Society.

At the same time, women are having children later in life, or choosing not to have children.

While having a baby may temporarily increase a woman’s breast cancer risk, it slightly lowers long-term risk, said Dr. Ann Partridge, co-founder and director of the Program for Young Adults with Breast Cancer at the Dana-Farber Cancer Institute.

Although a woman stops her menstrual cycle while pregnant, “it’s not just how many cycles a person has been exposed to, but it’s also about the timing of pregnancy, which is not related to estrogen,” Partridge said.

Instead, it may have something to do with changes that occur in the breast postpartum, when a woman’s mammary glands go back to a pre-pregnancy state. This elevated risk is higher among women who have babies later in life.

“Women who have children before age 30 have a decreased risk of developing breast cancer long term,” Partridge said. “We don’t know why, but the older you are when you have your first pregnancy, the higher-risk you are in the short term.”

Partridge added that it’s unclear why, but it may have to do with younger bodies typically being better at quelling inflammatory responses and repairing damaged DNA.

What’s known about breast cancer in younger women

Doctors are still scratching the surface in researching why breast cancer is becoming more prevalent at a younger age.

“Women are either not having children or having children later in life. They’re not breastfeeding as much, which does increase breast cancer risk. But that’s not all of it,” said Teplinsky. “We know that obesity and sedentary lifestyles contribute, but why all of a sudden are we seeing this increased risk?”

Teplinsky said more research is needed into the link between environmental toxins — such as chemicals known as endocrine disruptors — and breast cancer.

A study published earlier this year in the journal Frontiers in Toxicology found that nearly 200 chemicals linked to breast cancer are used in food packaging, including cardboard. Some recent studies have suggested using estrogen-only birth control can slightly raise a woman’s risk of breast cancer.

Obesity could be another driver of younger diagnosis of breast cancer, but the association is complicated. Obesity is believed to be one of several factors that together can trigger earlier puberty. It can also raise the risk of breast cancer by as much as 30%, but research suggests this is only in postmenopausal women, especially if a woman also has difficulty regulating blood sugar levels.

That would indicate that obesity likely isn’t much of a factor in the rising rates of breast cancer among younger women, said Toriola, of Washington University.

Toriola said most of the research being done on lifestyle factors and breast cancer is conducted on women in all age groups, which makes it difficult to tease out specific factors for younger women.

He added that more clinics should adopt risk-predictive models, such as the Tyrer-Cuzick risk assessment, that take into account more than just a woman’s personal and family breast cancer history and mutations, and include other factors such as early puberty.

“They’re a holistic overview of a woman’s risk of developing breast cancer,” Toriola said. “They take into consideration all the risk factors and would be a better indication of what actions and screening a woman would need to take.”