Risk factors for breast cancer
It is not known what causes breast cancer; however, several risk factors that may increase the chance of a woman developing breast cancer have been identified (see Box Breast cancer 1). Having a risk factor does not mean that a woman will get breast cancer – many women who have risk factors never develop the disease.
Box Breast cancer 1: Risk and protective factors for breast cancer
Breastfeeding
The only factor protective against breast cancer is breastfeeding (WCRF/AICR 2007).
Age
The greatest risk factor for breast cancer is age. Most breast cancers occur in women aged over 50 – in Australia, more than three‑quarters of breast cancers (AIHW 2024b).
Reproductive or hormonal factors
Certain reproductive or hormonal factors may increase a woman’s risk of developing breast cancer, including not having carried or given birth to any children (or to fewer children), older age at birth of first child, younger age at menarche, and older age at menopause. Oral contraception use can cause a small increase in the risk of breast cancer, as can hormone replacement therapy, which causes an increase in risk similar to that associated with late menopause (De et al. 2010; McPherson et al. 2000).
Family history and genetic susceptibility
A family history of breast cancer can increase a woman’s risk, although most women who develop breast cancer do not have this – 8 out of 9 women who develop it do not have a mother, sister or daughter with breast cancer (Breast Cancer Network Australia 2013).
Women with a BRCA1 or BRCA2 gene mutation (see Glossary) have a higher risk of developing breast cancer than the general population in all age groups. Women who carry a fault in BRCA1 or BRCA2 have a lifetime high risk of breast cancer: in a recent study, the cumulative breast cancer risk to age 80 was 72% for women who carry a fault in BRCA1 and 69% for women who carry a fault in BRCA2 (Kuchenbaecker et al. 2017).
Risk related to family history can be split into 3 categories (Cancer Australia 2015a):
- ‘At or slightly above average risk’ covers more than 95% of the female population and includes women with no family history as well as women with a weak family history (such as having one first degree relative diagnosed with breast cancer at age 50 or older). Nine out of 10 women in this group will not develop breast cancer.
- ‘Moderately increased risk’ covers less than 4% of the female population and includes women with a strong family history (for instance, one first degree relative diagnosed with breast cancer under the age of 50).
- ‘Potentially high risk’ covers less than 1% of the female population and includes women with a very strong family history (for instance, two first or second degree relatives on one side of the family diagnosed with breast or ovarian cancer plus one of a range of additional factors on the same side of the family, such as an additional breast cancer diagnosed before the age of 40, or breast cancer in a male relative).
Other factors
Other risk factors associated with breast cancer include a higher body mass index, exposure to X-rays and gamma radiation, and consumption of alcoholic beverages
(Cancer Research UK 2014).
Although most breast cancers occur in women over the age of 50, young women can (and do) develop breast cancer, as can men of any age, though more rarely. It is important, therefore, for people of all ages, including women, men, and non-binary people, to be aware of how their breasts normally look and feel and promptly report any new or unusual changes to their general practitioner.
AIHW (2024b) Cancer data in Australia, AIHW, Australian Government, accessed 15 August 2024.
Breast Cancer Network Australia (2013). Family history fact sheet. Melbourne: Breast Cancer Network Australia, accessed 11 August 2023.
Cancer Australia (2015a). Advice about familial aspects of breast cancer and epithelial ovarian cancer: a guide for health professionals. Sydney: Cancer Australia, accessed 11 August 2023.
Cancer Research UK (2014). Breast cancer risk factors. London: Cancer Research UK, accessed 11 August 2023.
De P, Neutel CI, Olivotto I & Morrison H (2010). Breast cancer incidence and hormone replacement therapy in Canada. Journal of the National Cancer Institute 102(19):1489–95.
Kuchenbaecker KB, Hopper JL, Barnes DR, Phillips KA, Mooij TM, Roos‑Blom MJ et al. (2017). Risks of breast, ovarian, and contralateral breast cancer for BRCA1 and BRCA2 mutation carriers. Journal of the American Medical Association 317(23):2402–16.
McPherson K, Steel CM & Dixon JM (2000). Breast cancer—epidemiology, risk factors, and genetics. British Medical Journal 321(7261):624–28.
UICC (Union for International Cancer Control) (2014) IARC release the latest world cancer statistics. Geneva: UICC. Viewed 11 August 2023.
WCRF/AICR (World Cancer Research Fund & American Institute for Cancer Research) (2007) Food, nutrition, physical activity, and the prevention of cancer: a global perspective. Washington DC: AICR.