Breast cancer develops when cells in breast tissue grow abnormally and form a tumor
Breast cancer starts when cells in the breast begin to divide uncontrollably. Most breast cancers begin in the cells that line the milk ducts or the lobules that produce milk. The exact reason a cell becomes cancerous is not fully understood, but researchers have identified factors that make the disease more likely to occur in some people than others.
No single cause explains why one person develops breast cancer and another does not. The disease usually results from a combination of factors: your genetics, your hormones, your age, and exposures over your lifetime. Understanding these factors can help you have informed conversations with your doctor about your personal risk.
Key Takeaways
- Breast cancer occurs when cells in breast tissue grow abnormally, most often starting in the milk ducts or milk-producing lobules.
- Age is the strongest single risk factor—most breast cancers occur in women over 50, though younger women can develop it too.
- Family history of breast cancer, especially in a mother or sister, raises your risk, particularly if the cancer occurred before age 50.
- Hormone-related factors including estrogen exposure, late menopause, early periods, and hormone replacement therapy increase risk for some women.
- Lifestyle factors like alcohol use, obesity, and not having children or breastfeeding are associated with higher risk, but many women with these factors never develop breast cancer.
Age and family history are the strongest known risk factors
Age is the single strongest risk factor for breast cancer. The risk rises steadily as you get older. Most breast cancers are diagnosed in women over 50, but breast cancer can occur at any age, including in younger women and men.
Family history matters significantly. If your mother, sister, or daughter was diagnosed with breast cancer, your risk is higher than average. The risk is even higher if that relative was diagnosed before age 50, or if multiple relatives on the same side of your family had breast cancer. Having a family history does not mean you will develop breast cancer—many women with a family history never do—but it does mean your risk is elevated compared to women without that history.
Some families carry inherited mutations in genes called BRCA1 or BRCA2. These mutations substantially increase the risk of breast cancer over a lifetime. A genetic counselor or your doctor can discuss whether genetic testing makes sense for your situation based on your family's cancer history.
Hormones and reproductive history affect breast cancer risk
Estrogen and progesterone, the hormones that regulate the menstrual cycle and pregnancy, play a role in breast cancer development. The longer a woman's lifetime exposure to estrogen, the higher her risk. This is why certain reproductive events and timing matter.
Women who started their periods before age 12 or went through menopause after age 55 had longer estrogen exposure over their lives and face higher risk. Women who never had children, or who had their first child after age 30, also have higher risk than women who had children earlier. Breastfeeding appears to lower risk slightly, possibly because it interrupts the menstrual cycle.
Hormone replacement therapy (HRT) used during and after menopause increases breast cancer risk for some women, particularly combined estrogen-progestin therapy used for several years or longer. The risk appears to decline after stopping HRT. Birth control pills also contain hormones, and current or recent use is associated with a modest increase in risk that decreases after stopping the medication.
Lifestyle factors and body weight influence risk
Alcohol consumption is linked to higher breast cancer risk. The risk increases with the amount of alcohol consumed—even moderate drinking (one drink per day) raises risk slightly compared to no alcohol. The mechanism is not completely understood, but alcohol may affect estrogen levels or damage DNA in breast cells.
Obesity after menopause increases breast cancer risk, likely because fat tissue produces estrogen. Before menopause, the relationship between weight and breast cancer risk is less clear. Physical activity appears to lower risk, independent of weight.
Smoking has not been conclusively linked to breast cancer in most studies, though some research suggests it may increase risk in certain groups. Exposure to radiation, particularly to the chest during medical imaging or cancer treatment in childhood or young adulthood, increases risk later in life.
Breast density and previous breast conditions matter
Breast density refers to the proportion of fibrous and glandular tissue compared to fatty tissue in the breast. Women with dense breast tissue have higher breast cancer risk than women with mostly fatty tissue. Dense tissue also makes cancers harder to detect on mammography, which is why some states require notification if your mammogram shows dense breasts.
A previous diagnosis of certain benign (non-cancerous) breast conditions raises risk. Atypical hyperplasia—abnormal cell growth that is not cancer—increases risk substantially. Lobular carcinoma in situ (LCIS) is considered a high-risk marker rather than cancer itself, but it indicates significantly elevated future risk. Other benign conditions like fibroadenomas or simple cysts do not increase risk.
Race, ethnicity, and other demographic factors
Breast cancer incidence and outcomes vary by race and ethnicity in the United States. White women have the highest incidence of breast cancer overall, but Black women are more likely to be diagnosed at younger ages and with more aggressive types. Black women also have higher mortality rates from breast cancer, influenced by both biological factors and differences in access to screening and treatment.
Hispanic, Asian, and Native American women generally have lower breast cancer incidence than white or Black women, though incidence is rising in some of these groups. These differences reflect a complex mix of genetic variation, reproductive patterns, obesity rates, and healthcare access rather than any single cause.
Men can develop breast cancer, though it is rare—accounting for about 1% of all breast cancers. Risk factors for men include family history, BRCA mutations, estrogen exposure, liver disease, and obesity.
What does not cause breast cancer
Several things once suspected of causing breast cancer have been studied extensively and are not confirmed as causes. Underwire bras, antiperspirants, and caffeine do not cause breast cancer. Abortion and miscarriage do not increase risk. Mammography itself does not cause breast cancer, though the radiation exposure is a consideration when weighing screening benefits and risks.
Stress has not been proven to cause breast cancer, though chronic stress may affect immune function and hormone levels in ways that theoretically could matter. Trauma to the breast does not cause cancer, though an injury might draw attention to a lump that was already present.
Frequently Asked Questions
If I have a family history of breast cancer, will I definitely get it?
No. Having a family history increases your risk, but most women with a family history of breast cancer never develop it. Your individual risk depends on how many relatives were affected, at what age they were diagnosed, and which side of your family the cancer occurred on. A doctor or genetic counselor can help you understand your specific risk level.
Can men get breast cancer?
Yes, though it is rare. Men account for about 1% of breast cancer cases. Risk factors include family history of breast cancer, BRCA gene mutations, obesity, liver disease, and estrogen exposure. Men with these risk factors should discuss screening and monitoring with their doctor.
Does having dense breast tissue mean I will get cancer?
No. Dense breast tissue increases your risk compared to women with fatty tissue, but most women with dense breasts never develop cancer. Dense tissue does make mammography less sensitive, so your doctor may recommend supplemental screening like ultrasound or MRI if you have dense breasts and other risk factors.
If I have a BRCA mutation, is breast cancer inevitable?
No. Women with BRCA1 or BRCA2 mutations have a substantially higher lifetime risk of breast cancer—around 45% to 87% depending on the mutation—but not all women with these mutations develop cancer. Options for managing this risk include more frequent screening, preventive medications, or preventive surgery. A genetic counselor can discuss what these options mean for your situation.
Does birth control increase my breast cancer risk?
Current or recent use of birth control pills is associated with a modest increase in breast cancer risk that decreases after stopping the medication. The increased risk is small compared to other factors like age and family history. The decision to use birth control involves weighing many health factors, and your doctor can discuss your individual risk profile.