It’s estimated one in eight women in Australia will develop breast cancer by the age of 85. While it’s impossible to determine what causes breast cancer in any individual woman, there are breast cancer risk factors that make it more likely in some women.
It’s not possible to determine what causes breast cancer in any individual woman. However, studies looking at very large numbers of women have shown there are some characteristics that are more common among groups of women who have developed breast cancer compared to groups of women who have not. These are called risk factors.
Having certain risk factors increases a woman’s chance of developing breast cancer. However, it doesn’t mean you will get breast cancer. And many women who develop breast cancer have no obvious risk factors.
Some of the risk factors for breast cancer include growing older, having a strong family history of the disease, being overweight, drinking alcohol and other lifestyle and environmental factors. Many of these factors are beyond your control, but there are some things you can change. There are also some factors that are thought to have a protective effect against breast cancer, like having children at a younger age and breastfeeding. However, women with protective factors may still develop breast cancer. Please note not all risk factors for breast cancer have been included in this resource.
Personal characteristics
- Gender – Being a woman is the main risk factor for breast cancer. Women are 100 times more likely to develop breast cancer than men. The rest of this summary relates to breast cancer risk for women. More information about breast cancer in men is available here.
- Age – Other than gender, increasing age is one of the strongest risk factors for developing breast cancer. Breast cancer can occur in younger women, but about three out of four breast cancer cases occur in women aged 50 years and older. About 0.5% of all breast cancers in Australia are diagnosed in women under 30 years. The average age of a breast cancer diagnosis in Australia is 61 years old.
- Height – Being taller as an adult is associated with a slightly increased risk for breast cancer. The risk of breast cancer increases by about 17% for each extra 10 cm in a woman’s adult height.
- Breast density – Increased breast density is associated with an increased risk of breast cancer. Your breast density is something that can only be seen on a mammogram and isn’t related to how breasts feel or how firm they are. A mammogram of a woman with dense breasts will appear much like white cotton wool. A mammogram of a woman with less dense breasts will appear more grey and transparent. The Breast Screening Australia’s position statement says: “There is no randomised controlled trial data that shows supplemental screening (such as MRI, ultrasound or tomosynthesis) saves additional lives for asymptomatic women with dense breasts and no other risk factors.” They recommend mammography as the best screening test for the early detection of breast cancer in all asymptomatic women, including women with dense breasts.
- Socioeconomic status – Breast cancer occurs more frequently in women who live in more affluent areas. This probably relates to lifestyle and reproductive factors. In addition, women of a higher socioeconomic status may have access to more screening services for early detection of breast cancer.
- Remoteness – Breast cancer occurs more frequently in women who live in more urban areas compared to those who live in rural or remote areas. This probably relates to lifestyle and reproductive factors. In addition, women in urban areas may have access to more screening services for early detection of breast cancer.
Family history
The significance of a family history of breast cancer increases with:
- the number of family members affected
- the younger their ages at diagnosis
- the closer the affected relatives are related to you.
The increase in risk is fairly small unless there are three or more first or second-degree relatives on the same side of the family with breast or ovarian cancer. It is important to note that a family history on your father’s side is just as important as it is on your mother’s side of the family.
The risk is stronger if two or more relatives have other characteristics associated with increased risk, such as being diagnosed before age 50 or being of Ashkenazi Jewish descent. Although women who have one or more first-degree relatives with a history of breast cancer are at increased risk, most will never develop breast cancer. Of those women with a family history who do develop breast cancer, most will be older than 50 years when their cancer is diagnosed.
Despite the importance of family history as a risk factor, eight out of nine women who develop breast cancer do not have an affected mother, sister, or daughter.
Inherited genetic factors – We all inherit a set of genes from each of our parents. Sometimes there is a fault in a copy of a gene which means that gene doesn’t work properly. That fault is called a mutation. When a person develops breast or ovarian cancer from inheriting that faulty gene, it’s called hereditary cancer. Several gene mutations may be involved in the development of breast or ovarian cancer. Some of the common ones are BRCA1 and BRCA2 genes. If a woman has a mutation in either of these genes, she has a higher chance of developing breast or ovarian cancer. It doesn’t mean she will definitely develop cancer though.
A woman with the faulty BRCA1 gene has about a 70% chance of developing breast cancer and about a 44% chance of developing ovarian/fallopian tube/primary peritoneal cancer over the course of her lifetime (up to 80 years of age). A woman with the faulty BRCA2 gene has about a 69% chance of developing breast cancer and about a 17% chance of developing ovarian/ fallopian tube/ primary peritoneal cancer over the course of her lifetime (up to 80 years of age).
However only 5% of breast cancers and up to 15% of epithelial ovarian cancers can be explained by an inherited gene fault.
The frequency of BRCA1 and BRCA2 mutations are seen more frequently in certain ethnic populations such as Ashkenazi Jewish (Jews whose origins can be traced back to Eastern Europe) population and in populations of Iceland, the Netherlands, Sweden, Hungary, Italy, South Africa and Pakistan. In Australia, it’s estimated that these gene faults occur in 2.5% of the Ashkenazi Jewish population compared with less than 1% of the general population.
Reproductive factors
- Age of first period – Women who started their first period when younger than 12 years old have a slightly higher risk of breast cancer than women who were 12 or older.
- Age at when first child was born – Being older at the birth of a woman’s first child has an increased risk of breast cancer. Women who had their first child aged 30 years old have about a 20% higher risk of breast cancer than a woman who had their first child aged 25-29.
- Not having children – Women who haven’t had children have about a 16% greater risk of breast cancer than women who have had at least one child.
- Age at menopause – Women who experience menopause later (at age 55 or after) have a higher risk of developing breast cancer. A woman’s risk of breast cancer increases by about 3% for each year older she is at menopause. So, a woman who goes through menopause 55 years or older has about a 12% higher risk of breast cancer than a woman aged 50–54 years.
- Naturally occurring hormones (endogenous) – If you are post-menopausal, having increased concentrations of naturally occurring estrogens or androgens is associated with higher breast cancer risk. There is also an increased breast cancer risk associated with naturally-occurring hormones testosterone and insulin-like growth factor 1 (IGF-1).
Lifestyle
- Weight – Women who are overweight or obese after menopause are at greater risk of developing breast cancer, and this risk increases with increasing body weight.
The reason for this isn’t fully understood, although it’s likely because fat tissue increases levels of the hormone estrogen in the body, which is a known risk factor for breast cancer.
For each 5-unit increase in BMI after menopause, the risk of breast cancer increases by about 12%.
However, in contrast, women with a higher BMI before menopause have a decreased risk of premenopausal cancer. For each 5-unit increase in BMI, the risk of premenopausal breast cancer decreases by about 7%. The link between body fatness and premenopausal cancer isn’t well understood.
- Weight gain after menopause – Gaining weight as an adult is also associated with increased risk of postmenopausal breast cancer. After menopause, the risk of breast cancer increases by about 6% for each 5kg increase in a woman’s weight.
- Alcohol – Drinking alcohol is associated with an increased risk of breast cancer in postmenopausal women and probably associated with an increased risk of breast cancer in premenopausal women as well. Women who drink one standard glass of alcohol each day have 7% higher risk than a woman who never drinks. The risk increases for each glass a day that a woman drinks.
Medical history and medications
- A previous breast condition – Being previously diagnosed with some types of benign breast disease or non-invasive breast conditions such as ductal carcinoma in situ (DCIS) or lobular carcinoma in situ (LCIS) are associated with an increased risk of developing invasive breast cancer.
- Exposure to high levels of ionising radiation – High dose ionising radiation in the chest region for treatment of Hodgkin lymphoma and some childhood cancers is associated with an increased risk of breast cancer.
- Hodgkin disease – Having a previous history of Hodgkin lymphoma is associated with an increased risk of breast cancer. It’s generally believed to be a consequence of radiation for Hodgkin disease however some studies have found an increased risk regardless of radiation treatment.
- Combined oral contraceptives –Taking the combined oral contraceptive pill is associated with a small increased breast cancer risk. The risk of breast cancer in women using the pill increases by 7% for every 5 years they’re on it however the risk goes down when they stop taking it.
- Menopausal hormone therapy (MHT) – MHT (also called combined hormone replacement therapy or combined HRT) that contains estrogen and progestogen is associated with an increased risk of breast cancer. Women who are currently using MHT have about 1.7 times the risk of breast cancer than women who have never used it. The risk increases while the woman is taking it and decreases after they stop treatment.
Breast cancer protective factors
- Regular physical activity – Active women of all ages are at reduced risk of breast cancer compared to women who do not exercise. Women who do large amounts of physical activity have approximately 13% lower risk of post-menopausal and 17% of premenopausal breast cancer.
- Having children – Having a child is associated with a decreased risk of breast cancer. For each child a woman has, her risk decreases by 7%. Having children at a younger age (below 30) is also associated with a lower breast cancer risk.
- Breastfeeding – It’s believed breastfeeding may be associated with a lower risk of breast feeding. The longer a woman breast feeds, the more her risk decreases. When a woman breastfeeds for 12 months, it’s equivalent to a 5% decrease in breast cancer risk.
- Diet – There may be decreased breast cancer risk from eating a diet that includes dairy products, foods high in calcium (including dairy products, dark green vegetables, some fortified soy products, and fish with edible bones), starchy and non-starchy vegetables and foods high in carotenoids (fruits and vegetables that are yellow, orange and red colour).
Unproven breast cancer risk factors
- Soy products – There is no conclusive evidence that eating phytoestrogens (plant estrogens) and soy products are associated with an increased risk of breast cancer. The small number of studies that have been done have produced inconsistent results.
- Pregnancy termination – A large number of high-quality studies have found no link between termination of pregnancy and increased risk of breast cancer. This includes both induced abortion and spontaneous miscarriage.
- Breast implants – Several studies have found no conclusive evidence that silicone breast implants increase a woman’s risk of developing breast cancer.
- Antiperspirant deodorant – There has not been enough research to determine whether anti-perspirant deodorant is associated with an increased risk of breast cancer. Deodorants may contain chemicals such as parabens and aluminum compounds which has led to a concern about the increased risk of breast cancer. However, no link has been found between these chemicals and breast cancer risk.
- Stress – The majority of research to date does not show a link between stress and increased risk of breast cancer. However, stress can lead to changing processes in the body such as in the immune system and can also lead to lifestyle changes such as drinking more alcohol. These factors could increase breast cancer risk.
- Underwire Bras – Research does not support the claim that underwire or tight-fitting bras increase a woman’s risk of breast cancer.
- A bump to the breast – Research has shown a bump or knock to the breast does not cause breast cancer. However, it can draw attention to an existing lump in the breast.
