Breast cancer is likely to be caused by a combination of different risk factors. For example, getting older is one of the main risk factors for breast cancer.

The exact cause of breast cancer is unknown. But certain things can increase your chance of developing it. These are called risk factors. The risk factors for invasive breast cancer and ductal carcinoma in situ (DCIS) are similar.

Having one or more risk factors does not mean you will definitely get breast cancer. And if you do not have any risk factors, it does not mean you will not get it.

Breast cancer is likely to be caused by a combination of different risk factors, rather than just one.

Age

The strongest risk factor for invasive breast cancer is increasing age. About 8 out of 10 women diagnosed (80%) are over the age of 50. Breast cancer is rare in women under 30.

If you have had breast cancer before

Your risk of developing invasive breast cancer is increased if you have had breast cancer or ductal carcinoma in situ (DCIS) before.

In this case, you will have regular follow-up appointments. Any changes in the same breast or the other breast can be checked quickly.

Hormonal factors

The female hormones oestrogen and progesterone can affect your breast cancer risk. Factors that can increase your risk include the following:

• Taking hormone replacement therapy (HRT) for more than 5 years, especially if you are taking combined HRT (oestrogen and progesterone). When you stop HRT, your risk reduces again.
• Not having had children.
• Having had your first child after the age of 30.
• Not breastfeeding your children, or breastfeeding for less than a year in total.
• Starting your periods early (under the age of 12) or having a late menopause (after the age of 55).
• Taking the contraceptive pill. But the risk reduces if you stop taking it.

Family history

Most women who get breast cancer do not have a family history of it. Or if you have only one female relative diagnosed with breast cancer over the age of 40, your risk is unlikely to be very different from other women the same age as you. But sometimes breast cancer can run in families. The chance of there being a family link is bigger when:

• a number of family members have been diagnosed with breast cancer or related cancers, such as ovarian cancer
• the family members are closely related
• the family members were diagnosed at a younger age
• a man in your family has been diagnosed with breast cancer.

Fewer than 1 in 10 breast cancers (10%) are thought to be caused by a change (alteration) in a gene running through the family. In hereditary breast cancer, BRCA1 and BRCA2 are the two genes most often found to have a change.

Women with triple negative breast cancer are sometimes offered genetic testing. This is offered even if they do not have a family history of breast cancer. Most breast cancers caused by a change in the BRCA1 gene are triple negative. Your doctor or breast care nurse can explain more about this to you.

If you are worried about breast cancer in your family, talk to your GP or breast specialist. They can refer you to a family history clinic or a genetics clinic.

Lifestyle factors

Certain lifestyle factors may slightly increase your breast cancer risk.

Being overweight

The risk of breast cancer is higher in women who are overweight, particularly after the menopause. This is because being overweight may change hormone levels in the body. Keeping to a healthy weight can help reduce the risk of breast cancer.

Alcohol

Regularly drinking alcohol increases your risk of developing breast cancer. But the risk is small for women who drink within the recommended guidelines.

Smoking

Smoking may cause a slight increase in breast cancer risk. This seems to be linked with starting smoking at a younger age and smoking for a longer time.