Breast cancer develops when cells in the breast grow abnormally and divide uncontrollably, and no single cause explains every case. About 13% of women will be diagnosed with breast cancer at some point in their lifetime. The disease results from a combination of genetic, hormonal, lifestyle, and environmental factors, and understanding these can help you assess your own risk.
Inherited Gene Mutations
The most well-known genetic risk factors are mutations in the BRCA1 and BRCA2 genes. These genes normally help repair damaged DNA in your cells. When they carry a harmful mutation, that repair system breaks down, and cells are more likely to accumulate errors that lead to cancer. More than 60% of women who inherit a harmful change in either BRCA1 or BRCA2 will develop breast cancer during their lifetime. Men with these mutations also face elevated risk: about 0.2% to 1.2% with a BRCA1 change and 1.8% to 7.1% with a BRCA2 change will develop breast cancer by age 70.
BRCA mutations get the most attention, but they aren’t the only inherited genes involved. A mutation in the PALB2 gene carries a 53% lifetime risk of breast cancer by age 80, roughly seven times higher than the general population. Other genes like CHEK2, ATM, and TP53 also raise risk to varying degrees. Collectively, inherited mutations account for 5% to 10% of all breast cancers. The remaining cases arise from genetic changes that happen during a person’s life rather than being inherited from a parent.
How Hormones Drive Breast Cell Growth
Estrogen and progesterone are the hormones most directly linked to breast cancer risk. They work by entering breast cells, binding to receptors, and switching on genes that tell cells to divide. This is a normal part of breast development and menstrual cycling, but more lifetime exposure to these hormones means more rounds of cell division, and each division carries a small chance of a DNA copying error.
Estrogen’s role goes beyond just stimulating growth. It can cause replication stress in cells, leading directly to DNA damage. When that damage isn’t properly repaired, it can trigger the mutations that initiate cancer. This is why factors that increase your total lifetime estrogen exposure, like starting periods early (before age 12), reaching menopause late (after 55), or never having children, all raise breast cancer risk. Each of these extends the window during which breast tissue is repeatedly exposed to hormonal growth signals.
Body Weight After Menopause
Before menopause, your ovaries are the main source of estrogen. After menopause, ovarian production stops, but fat tissue picks up the slack. Fat cells contain an enzyme called aromatase that converts other hormones into estrogen. In women with more body fat, aromatase activity is higher, and there are simply more fat cells producing estrogen. This creates a strong causal relationship between obesity and estrogen receptor-positive breast cancer in postmenopausal women.
The effect compounds in another way: as breast fat tissue expands, the number of cells capable of producing estrogen within the breast itself increases. So excess weight doesn’t just raise estrogen levels in your blood. It raises estrogen levels right where breast tumors are most likely to form. This localized hormone production can promote cell proliferation and suppress the natural cell death process that normally keeps abnormal cells in check.
Alcohol Consumption
Alcohol is one of the most clearly established modifiable risk factors for breast cancer. A pooled analysis of more than one million women found that drinking up to about one alcoholic beverage per day raised breast cancer risk by 10% compared to nondrinkers. Women consuming more than two drinks per day faced a 32% increase in risk. The effect is dose-dependent, meaning more alcohol equals more risk, with no clear safe threshold identified.
Alcohol raises risk through several pathways. It increases circulating estrogen levels, generates a toxic byproduct called acetaldehyde that can damage DNA directly, and may impair the body’s ability to absorb nutrients that protect against cancer. Unlike some risk factors you can’t change, alcohol intake is entirely within your control.
Reproductive History and Breastfeeding
Women who have children generally have lower breast cancer risk than those who don’t, and the age at which you have your first child matters. A first full-term pregnancy at a younger age appears to offer more protection, likely because pregnancy causes breast cells to mature into a more stable state that’s less vulnerable to cancer-causing mutations.
Breastfeeding adds further protection. A landmark study across multiple countries found that breast cancer risk drops by about 4.3% for every 12 months a woman breastfeeds. Part of this benefit comes from breastfeeding suppressing ovulation, which reduces lifetime estrogen exposure. Part of it comes from the breast shedding cells at the end of lactation, which may clear out cells that have accumulated DNA damage.
Hormone Therapy for Menopause
Combined hormone therapy, which uses both estrogen and a progestin to manage menopausal symptoms like hot flashes, increases breast cancer risk and deaths from the disease. This was established by the Women’s Health Initiative, one of the largest clinical trials ever conducted on the subject. Whether estrogen-only therapy (used by women who have had a hysterectomy) carries the same risk is less clear: some large studies found little additional risk, while others found a statistically significant increase.
The reassuring finding is that risk drops quickly after stopping combined hormone therapy. Breast cancer incidence rates declined rapidly once women in the studies discontinued treatment, returning close to baseline levels. If you’re weighing the benefits and risks of hormone therapy for menopause symptoms, the duration of use is a key factor in how much your risk increases.
Breast Density
Breast density refers to the proportion of fibrous and glandular tissue versus fatty tissue in your breasts, visible on a mammogram. Women with extremely dense breasts face 1.8 to 6 times the breast cancer risk compared to women with the least dense (mostly fatty) breasts. Dense tissue contains more cells that can potentially become cancerous, and it also makes tumors harder to spot on mammograms because both dense tissue and tumors appear white on the images.
Breast density is largely determined by genetics, age, and hormonal factors. It tends to decrease after menopause. Many states now require that mammogram results include information about your breast density, so you can discuss additional screening options with your care team if yours is high.
Radiation Exposure
Exposure to ionizing radiation, particularly to the chest area during childhood or adolescence, is a known breast cancer risk factor. This is most relevant for people who received radiation treatment for conditions like Hodgkin lymphoma at a young age, when breast tissue is still developing. The younger the exposure, the higher the risk, because developing cells are more susceptible to radiation-induced DNA damage.
Routine diagnostic imaging like mammograms uses very low doses of radiation and is not considered a significant risk factor. The concern is specifically about high-dose or repeated radiation exposure to the chest, particularly before age 30.
Factors You Can and Can’t Control
Some breast cancer risk factors are fixed: your genetics, your family history, your age at first period, and your breast density. You can’t change these, but knowing about them helps you make informed decisions about screening frequency and timing.
The factors you can influence include alcohol intake, body weight (especially after menopause), physical activity, breastfeeding, and decisions about hormone therapy. No single lifestyle change eliminates risk entirely, and many women who do everything “right” still develop breast cancer. But the modifiable factors are meaningful in aggregate. Maintaining a healthy weight after menopause, limiting alcohol, staying physically active, and breastfeeding if possible collectively lower your odds in a measurable way.