Breast Cancer Risk Factors

Many women who develop breast cancer do not have any known risk factors. Still, we know that women who possess certain risk factors are at a higher risk of developing breast cancer than the general population. Although some women who have one or more risk factors may never develop breast cancer, we can use the knowledge of these risk factors to target higher-risk women with increased breast surveillance and breast cancer prevention strategies.

Certain, unavoidable risk factors — such as gender and age — make us all susceptible to breast cancer. Other risk factors, such as family history, are also factors that we cannot change. However, research has shown that there are some risk factors, including alcohol intake and body weight, which are modifiable.

Below you will find a summary of the factors that increase risk for developing breast cancer, including both factors that we cannot change and those we can.

Risk Factors We Cannot Change


Women account for more than 99 percent of all breast cancer cases.


After gender, age is the most influential risk factor for developing breast cancer. Women younger than age 40 account for only 4.7 percent of invasive breast cancer diagnoses and only 3.6 percent of in situ breast cancer diagnoses. Over 70 percent of all breast cancer diagnoses are made in women who are 50 or older.

You may have heard the statistic that one in eight women in the United States will develop breast cancer. This does not mean that a woman of any age has a one-in-eight chance of developing breast cancer. This statistic conveys a woman's lifetime risk. This means that if a woman lives until age 85, she has a risk of one in eight of developing breast cancer sometime during her lifetime.


After age 40, Caucasian women are more likely to be diagnosed with breast cancer than African-American women. However, African-American women are more likely than white women to die of breast cancer. Women of Asian, Hispanic or American Indian descent are at lower risk than Caucasian or African-American women for developing breast cancer.

Personal History

If a woman has had cancer in one breast, she is at increased risk of developing cancer in the other breast.

Family History

Women with a relative who has had breast cancer are at higher risk of developing breast cancer themselves, particularly if it is a first-degree relative, such as a mother, sister or daughter.

That risk is further increased if a woman has multiple first-degree relatives who have had breast cancer, or if she has a first-degree relative who developed breast cancer at a young age or in both breasts.

Patients with family members who have had breast and/or ovarian cancer may choose to see a qualified genetic counselor from the UCSF Cancer Genetics and Prevention Program at Mount Zion. These counselors are available to evaluate a person's likelihood of carrying a gene mutation and to discuss the possibility of genetic testing.

Genetic Risk Factors

Women who have certain inherited gene mutations (including BRCA1 and BRCA2) have a significantly increased risk of breast cancer and account for about 5 percent to 10 percent of breast cancer cases. In most women, the normally functioning BRCA1 and BRCA2 genes help to prevent breast cancer by controlling cell growth. However, these genes are no longer able to control cell growth properly unmutated.

Since these genes are passed down from your parents, it is possible to carry a gene mutation from the mother or father's side of the family. A female who carries either the BRCA1 or BRCA2 gene mutation has up to an 85 percent chance of developing breast cancer by the age of 70. However, in men the BRCA2 gene mutation is reported to increase risk of breast cancer more than the BRCA1 gene mutation. Males who carry the BRCA2 gene mutation have a suggested 6 percent chance of developing breast cancer during a lifetime.

A prevalence of the BRCA1 and BRCA2 gene mutations has been observed in the Ashkenazi Jewish (Jews with European or Central European ancestry) population. Having one or more relatives with breast or ovarian cancer, and being of Ashkenazi Jewish descent, puts a person at greater risk for carrying a BRCA gene mutation.


Exposure to high doses of chest radiation, such as for medical therapy for Hodgkin's lymphoma, particularly during childhood, can greatly increase a woman's risk of developing breast cancer.

Researchers have found that the age at which radiation was received is inversely related to the acquired risk. Thus, women who received radiation after their menopausal years incurred very little risk.

DES Exposure

In the 1950s and 1960s, many pregnant women took a synthetic form of estrogen called diethylstilbestrol (DES) to prevent miscarriage. Many of these women's daughters eventually developed vaginal and cervical cancer at a rate that seemed higher than normal, and studies found that DES exposure was indeed associated with an increased risk of these types of cancer.

Because of the exposure to additional estrogen, women who were exposed to DES in utero also may be at higher risk for breast cancer. A study published in October 2002 found that in women who were 40 years and older, breast cancer risk was in fact increased if a woman had been exposed to DES.

Age at Menstruation

A woman's amount of exposure to estrogen and progesterone during her lifetime is believed to be a risk factor. The longer a woman is exposed, the more likely she is to develop breast cancer. Therefore, if a woman begins menstruation before age 12, she is believed to be at slightly higher risk.

Age at First Birth

It has been observed that women who have their first child after age 29, or who do not have any children, are at slightly higher risk for breast cancer than women who have their first child before age 29. It has been proposed that breast changes during pregnancy may have protective effects against cancer development because risk of breast cancer appears to decrease with each additional childbirth.

It is important to note that evidence suggests the opposite is true for women who have a family history of breast cancer. In other words, women who have a family history of breast cancer are at lower risk if they have no children or have their children at a later age.

Age at Menopause

Women who go through menopause after the age of 54 have a slightly higher risk of breast cancer than women who go through menopause at age 54 or younger. Their higher risk may be related to their higher lifetime exposure to estrogen and progesterone.

Atypical Hyperplasia or Atypia

Either atypical hyperplasia or atypia indicates the growth of abnormal cells in the breast. The diagnosis of atypical hyperplasia can be made from a core biopsy or excisional biopsy, and has been correlated with an increased risk of breast cancer.

The diagnosis of atypia can be made from nipple aspiration, ductal lavage, or fine needle aspiration (FNA), and also indicates an increased breast cancer risk. Although these cells are not yet cancerous, they do raise a woman's risk of eventually developing breast cancer. While biopsies and FNAs are usually reserved for when there is a current indication that a woman might have breast cancer, nipple aspiration and ductal lavage are methods that may help assess a woman's future risk of breast cancer.

Breast Density

Studies have consistently shown that higher breast density is linked with increased risk of breast cancer. Research is examining whether breast density may be modifiable by changing women's hormones or diet. One medication that has been demonstrated to reduce breast density is tamoxifen.

Serum Estradiol Level

Estradiol is the predominant form of estrogen circulating in the body. 'Serum estradiol' refers to the amount of estradiol in the blood, so a woman's level of serum estradiol may be measured with a simple blood test.

In postmenopausal women, higher hormone levels in the blood have been associated with an increased risk of breast cancer.

Risk Factors We Can Change


Studies have shown a clear association between obesity and increased risk of post-menopausal breast cancer. Because having more fat tissue can increase a woman's level of estrogen, it is important for a woman to attempt to control her weight, particularly after menopause. Once a woman has stopped menstruating, her levels of estrogen and progesterone are much lower than they once were. Excess fat tissue may cause significant increases in her hormone levels.

Physical Activity

Physical activity not only helps a woman reduce her risk of breast cancer by maintaining a healthy body weight, it may also have its own benefits to risk reduction. Some studies have shown that physical exercise throughout a woman's life reduces her risk, independent of her weight.

One theory is that exercise may reduce a woman's risk by limiting menstrual function, and it has been observed that regular physical exercise can delay menarche — the onset of menstruation — when body fat percentage is low. Since breast cancer risk may be significantly influenced by a woman's lifetime exposure to hormones, reducing that exposure may also reduce her risk.

Alcohol Consumption

Many epidemiological studies spanning the past 20 years have shown an association between alcohol consumption and increased risk of breast cancer. Studies have consistently found that women who consume at least three alcoholic drinks per day are at higher risk for developing breast cancer than women who do not drink alcohol. Furthermore, study findings have shown that for women who drink two alcoholic drinks or more per day, breast cancer risk is related to the amount of alcohol consumed — higher consumption of alcohol equals higher breast cancer risk.

Among other mechanisms, it has been suggested that alcohol may increase a woman's hormone levels. A recent study fed women specified amounts of alcohol each day, and demonstrated that a woman's levels of blood estrogen did increase according to the amount of alcohol she consumed. In particular, the breast cancer risk of post-menopausal women, whose bodies make very little estrogen compared with pre-menopausal women, may be affected by alcohol consumption by this mechanism.

Hormone Replacement Therapy

A study conducted by the Women's Health Initiative (WHI) showed that women in the study population who took hormone replacement therapy — combined estrogen and progestin — had a 26 percent increased risk (relative to an average woman) of invasive breast cancer after four to five years of therapy. This finding is consistent with the growing evidence that exogenous (outside) hormones increase a woman's lifetime estrogen exposure as well as increasing her breast cancer risk.


An analysis published in July 2002 pulled together data from 47 previous studies to show that breastfeeding does in fact slightly lower a woman's risk of breast cancer. The longer a woman breastfed, the lower her risk was. For optimal benefit, we recommend breastfeeding a child for 12 months.

The decision of whether or not to breastfeed is certainly a very personal one. The knowledge that breastfeeding may offer a slight reduction in risk for developing breast cancer is just one of the many factors that will influence how long a woman decides to breastfeed.

Factors Requiring Further Study

Oral Contraceptives

The effect of oral contraceptives (birth control pills) on breast cancer risk is still being studied. While some studies have shown that taking oral contraceptives slightly increases a woman's risk of breast cancer, other studies have shown no effect on risk.

A recent analysis showed that women who took oral contraceptives in the long term — for more than 12 years — had a slightly higher risk of breast cancer than women who did not take oral contraceptives. Once women stopped taking oral contraceptives for 10 years, their risk appeared to return to the baseline, average risk.

Environmental Pollutants

Because studies have shown that breast cancer is not always attributable to inherited factors, extensive research is examining aspects of the environment that might contribute to breast cancer development. Some studies are focused on a possible link between environmental pollutants, such as pesticides, and an increased risk of breast cancer, but no clear link has been established.

Thus far, the evidence that has been gathered suggests that environmental pollutants are probably not the major cause of breast cancer.


We know that smoking increases a person's risk of heart disease and lung cancer, but a consistent association between smoking and an increased risk of breast cancer has not been demonstrated. However, researchers continue to study the potential impact of smoking on breast cancer risk and, in some cases, they are noticing a link.

A recent study suggested that women who were exposed to cigarette smoke, both through active and passive smoking, were indeed at higher risk of breast cancer. Another study, published in the same month, proposed that the effect of cigarette smoking on breast cancer risk was related to the time of exposure in its study population, with women who had started smoking at a young age having the highest risk.

Although the association between smoking and breast cancer risk is not clearly established, smoking is strongly discouraged because of its known impact on a person's risk of heart disease and lung cancer.

High-Fat Diet

Several studies have investigated the relationship between a high-fat diet and a woman's risk of breast cancer. While some studies have shown that a high-fat diet does increase a woman's breast cancer risk, other studies have not found a significant relationship. Researchers also are examining whether the types of fat eaten affect the risk of breast cancer, such as saturated versus unsaturated fat.

Thus far, while there is a clear link between a high-fat diet and an increased risk of heart disease, the association between diet and breast cancer risk is unclear. Nonetheless, it is important to manage dietary fat intake in order to maintain a healthy body weight.

Factors with No Proven Impact on Risk


No research has shown that the use of antiperspirant increases the risk of breast cancer. A recent study showed that women who had breast cancer were no more or less likely to use antiperspirant than women who had not had breast cancer.

Underwire Bras

Rumors over e-mail have suggested that underwire bras might cause breast cancer. However, there is absolutely no evidence demonstrating that underwire bras are related to the development of breast cancer.


Because of the hormonal changes during pregnancy, many studies have examined the possible relationship between abortion and breast cancer risk. Some studies have shown that induced abortion may result in a slight increase in risk, but many studies have shown no effect on risk, including a study of 1.5 million women in Denmark.

At this time, there is insufficient data to suggest that women who have had abortions are at increased risk.

Breast Implants

Studies have consistently shown that breast implants are not associated with increased rates of breast cancer. Women with breast implants should follow breast screening guidelines just like women without breast implants. The only difference is that women with breast implants will have additional images taken during their mammograms to ensure that all the breast tissue is available for observation.


Reviewed by health care specialists at UCSF Medical Center.

This information is for educational purposes only and is not intended to replace the advice of your doctor or health care provider. We encourage you to discuss with your doctor any questions or concerns you may have.