What causes breast cancer?

Exactly why breast cancer develops in a particular woman is not clear.

Breast cancer seems to be linked to hormones, especially the female hormone oestrogen. However, it is unlikely there is one single cause. A combination of factors, some known but others unknown, may trigger or promote the cancer.

Risk factors and protective factors

At present there are no certain ways of preventing breast cancer although there are some clues - known as risk factors and protective factors - about who is more likely or less likely to develop the disease. Many of these risk factors are linked to female hormones, especially oestrogen, for example, age at puberty, age at first pregnancy and age at menopause. Many risk and protective factors are uncertain or controversial.

The risk factors for ductal carcinoma in situ (DCIS), an early form of breast cancer that is contained within the breast duct and has not spread into the surrounding tissue, appear to be similar to those for invasive breast cancer.  DCIS is diagnosed the same way as invasive breast cancer and treatment is offered to prevent the development of invasive breast cancer.

Most studies of risk factors have been done in women of European background, and risk factors may differ for women of different ethnicities.

The risk factors listed below are common among women, but there is little they can do about most of them. Some relate to our lives many years before. A few risk factors provide the opportunity to reduce risk by making changes in our lives, but even making those changes cannot give a guarantee.

Women who have the key risk factors should discuss this with their doctor who can advise them and, if necessary, develop a plan for regular checks that may include mammograms.

Factors that increase the risk of breast cancer

Key risk factors

  • Being female
  • Growing older
  • Previous breast cancer
  • Previous breast biopsy showing a condition that increases risk
  • Strong family history of breast and/or ovarian cancer
  • Inherited genetic factors
  • Exposure to repeated or high-dose radiation

Less important risk factors

  • Obesity post menopausally
  • Weight gain after 18
  • Current drinking of alcohol
  • Never had children
  • First child after 35
  • Hormone replacement therapy
  • Oral contraceptives
  • Depo-Provera contraceptive injection
  • Being a twin

No clear evidence*

  • High-fat, red meat diet
  • Environmental chemicals, such as pesticides
  • Heavy smoking and passive smoking
  • Use of statins – medicines to control cholesterol

(*No clear evidence means that the results of studies have been mixed, or there is not enough evidence to say that it is proven.)

No evidence

  • Deodorants
  • Electric blankets
  • Hair colouring
  • Abortion or miscarriage
  • Tea or coffee
  • Underwire bras
  • Bruise or injury to the breast
  • Personality type
  • Stress
  • Cellphones, digital clocks, microwaves

Factors that are protective

Protective factors

  • Menstruation starts at late age
  • Menopause occurs at a young age
  • First child at a young age
  • Having children, the more children the greater the protection
  • Breastfeeding

No clear evidence*

  • Low fat, high-fibre diet
  • Phyto-oestrogens - plant oestrogens
  • Exercise
  • Green tea
  • Regular use of non-steroidal anti-inflammatory drugs

(*No clear evidence means that the results of studies have been mixed, or there is not enough evidence to say that it is proven.)

Page last updated: 27 November 2014
0800 270 200