What Causes Breast Cancer?

Over the years, experts have discovered activities, medications, and other factors that may increase your risk of developing breast cancer.

Weight Women who are overweight after menopause may have an increased risk of breast cancer, according to the CDC. Before menopause, most of a woman’s estrogen is released from their ovaries. After menopause, however, the main source of estrogen in the body becomes fat tissue, and the more fat tissue present, the more estrogen is released, the ACS notes. High estrogen has been linked to breast cancer, according to a study published in 2023 in Nature, and so an increase in fat deposits can increase risk.

Inactive lifestyle Though experts aren’t sure why, regular exercise seems to reduce breast cancer risk, especially in post-menopausal women, according to the ACS. Even though the connection hasn’t been fully discovered, the ACS states the lowered risk could be due to weight loss, lower inflammation, and exercise-induced hormonal changes.

Hormone supplements Supplemental estrogen, sometimes combined with another hormone called progesterone, can be prescribed to treat symptoms of menopause. Some studies show hormone therapy may increase your risk of breast and other cancers, while other studies can’t find the connection, according to the ACS.

Estrogen therapy may lower the risk of osteoporosis and colorectal cancer, but the benefits should be weighed with the risks of not only breast cancer, but also blood clots, heart disease, and stroke, the ACS notes. Your healthcare provider can help you navigate the complex pros and cons of hormone supplements.

Birth control Birth control pills have been associated with a slightly higher risk of breast cancer. Oral birth control methods use hormones which may be the source of the correlation, the ACS states, and risk goes down 10 years after you stop taking the pills. Although few studies have been conducted to measure risk for implants, patches, and rings, they may also increase your risk.

Breastfeeding Choosing not to breastfeed may increase your risk of breast cancer, while breastfeeding for a year or more may decrease risk, according to a study published in September 2022 in Cancer Medicine. Although studies in the United States are lacking in this area, experts think the slightly lower risk may be related to fewer lifetime menstrual cycles, per the NCI.

Alcohol Breast cancer risk can go up with any amount of alcohol consumption, according to the NCI. One large study review published in Alcohol Research found that for every 10 grams (g) of pure alcohol consumed per day, the risk of breast cancer increased by 5 percent for premenopausal women and 9 percent after menopause. Breast cancer risk from alcohol consumption is cumulative, so if you decrease your intake, you can lower your risk, according to one study.

For reference, one beer contains 14 g of pure alcohol, according to the NCI. The ACS recommends women avoid any alcohol intake if possible. If you choose to drink, they recommend no more than one drink per day.

Unpreventable Risk Factors

This group of breast cancer risk factors arises not from your lifestyle choices, but from genetic predisposition, health history, age, and race. While these risk factors cannot be prevented, they can help you and your healthcare provider make a plan for screening, so you can lower your risk in other ways and catch cancer growth early.

Sex at birth Being born female is the highest unpreventable risk factor for breast cancer, the NCI states. While men can also develop breast cancer, they account for less than 1 percent of all new cases, according to a study published in Scientific Reports.

Age As women age, their risk of breast cancer increases. For example, a 30-year-old woman’s chance of developing breast cancer before she turns 40 is about 1 in 200, whereas a 70-year-old woman’s 10-year risk is 1 in 25, the NCI reports. Even so, the rate of breast cancer diagnosis in women younger than 40 has been on the rise since 2004, according to a clinical review published in JCO Oncology Practice.

Height A few studies have shown a link between being taller and having a higher risk of breast cancer, according to the ACS. Though experts aren’t sure exactly why, they think the connection could be related to nutrition in early life, and genetic and hormonal factors, according to a study published in Breast Cancer Research and Treatment.

Reproductive history If you had your first menstrual period before age 12 or began menopause after age 55, the increased exposure to hormones like estrogen may raise your risk of breast cancer, according to the CDC. More lifetime periods mean more estrogen exposure, which increases your breast cancer risk, according to the ACS.

Pregnancy Pregnancy and breast cancer have a complex relationship. While breast cancer risk rises for 10 years after the birth to your first child, pregnancy after the age of 30 or no history of pregnancy may also increase your risk because of a higher number of menstrual periods and exposure to more estrogen, according to the ACS.

Dense breasts Some women have greater breast density than others, and this arrangement of tissue can make tumors difficult to identify on a mammogram. Women with more glandular and fibrous tissue than fat have dense breast tissue. You may have denser breasts if you are pregnant, breastfeeding, taking hormone supplements, or are younger, the CDC states.

A higher breast density can be genetic, but can also be more common in women who have had no children, have their first pregnancy after age 35, take hormonal therapy, or consume alcohol, according to the NCI. Your provider may order a 3D mammogram, with or without an ultrasound, to identify tumors more effectively. Women with very dense breast tissue may also need breast MRIs in addition to regular mammograms.

Personal history Women have a higher risk of breast cancer diagnosis if they have already had breast cancer or certain noncancerous breast diseases — like atypical ductal hyperplasia or lobular carcinoma in situ — according to the CDC.

Previous treatment Radiation therapy to the chest or breast areas before the age of 30 increases the risk of developing breast cancer later on, notes the CDC. This therapy uses high doses of radiation to kill cancer cells, but it can also affect nearby healthy cells, according to the NCI.

That risk goes up about 10 years after treatment ends, and depends on the dosage used and the age administered, according to the NCI, which also notes the risk is highest when radiation is received during puberty.

Race and ethnicity Non-Hispanic white women are more likely to be diagnosed with breast cancer than Black women, but in women under the age of 45, the reverse is true, according to Cancer.Net. Black women are more likely to die of breast cancer than white women, no matter their age. Hispanic and Asian or Pacific Islander women have had lower mortality rates than white and Black women, but between 2016 and 2020, the mortality rate for American Indian or Alaska Native women surpassed that of white women, according to the ACS.