The causes of breast cancer aren't fully understood, making it difficult to say why one woman may develop breast cancer and another may not.

However, there are risk factors that are known to affect your likelihood of developing breast cancer. Some of these you can't do anything about, but there are some you can change.

Age

The risk of developing breast cancer increases with age. The condition is most common among women over 50 who have been through the menopause. About 8 out of 10 cases of breast cancer occur in women over 50.

All women who are 50-70 years of age should be screened for breast cancer every three years as part of the NHS Breast Screening Programme. Women over the age of 70 are still eligible to be screened and can arrange this through their GP or local screening unit. Read more about breast screening.

Family history

If you have close relatives who have had breast cancer or ovarian cancer, you may have a higher risk of developing breast cancer. However, because breast cancer is the most common cancer in women, it's possible for it to occur in more than one family member by chance.

Most cases of breast cancer aren't hereditary (they don't run in families), but particular genes, known as BRCA1 and BRCA2, can increase your risk of developing both breast and ovarian cancer. It's possible for these genes to be passed on from a parent to their child. A third gene (TP53) is also associated with increased risk of breast cancer.

If you have, for example, two or more close relatives from the same side of your family, such as your mother, sister or daughter, who have had breast cancer under the age of 50, you may be eligible for surveillance for breast cancer or for genetic screening to look for the genes that make developing breast cancer more likely. If you're worried about your family history of breast cancer, discuss it with your GP.

Read about predictive genetic tests for cancer risk genes.

Previous diagnosis of breast cancer

If you've previously had breast cancer or early non-invasive cancer cell changes in breast ducts, you have a higher risk of developing it again, either in your other breast or in the same breast again.

Previous benign breast lump

A benign breast lump doesn't mean you have breast cancer, but certain types of lump may slightly increase your risk of developing it. Certain benign changes in your breast tissue, such as atypical ductal hyperplasia (cells growing abnormally in ducts), or lobular carcinoma in situ (abnormal cells inside your breast lobes), can make getting breast cancer more likely.

Breast density

Your breasts are made up of thousands of tiny glands (lobules), which produce milk. This glandular tissue contains a higher concentration of breast cells than other breast tissue, making it denser. Women with dense breast tissue may have a higher risk of developing breast cancer because there are more cells that can become cancerous.

Dense breast tissue can also make a breast scan (mammogram) difficult to read, because it makes any lumps or areas of abnormal tissue harder to spot. Younger women tend to have denser breasts. As you get older, the amount of glandular tissue in your breasts decreases and is replaced by fat, so your breasts become less dense.

Exposure to oestrogen

The female hormone, oestrogen, can sometimes stimulate breast cancer cells and cause them to grow. The ovaries, where your eggs are stored, begin to produce oestrogen when you start puberty, to regulate your periods.

Your risk of developing breast cancer may rise slightly with the amount of oestrogen your body is exposed to. For example, if you started your periods at a young age and experienced the menopause at a late age, you'll have been exposed to oestrogen over a longer period of time. In the same way, not having children, or having children later in life, may slightly increase your risk of developing breast cancer because your exposure to oestrogen is uninterrupted by pregnancy.

Being overweight or obese

If you've experienced the menopause and are overweight or obese, you may be more at risk of developing breast cancer. This is thought to be linked to the amount of oestrogen in your body, because being overweight or obese after the menopause causes more oestrogen to be produced.

Being tall

If you're taller than average, you're more likely to develop breast cancer than someone who's shorter than average. The reason for this isn't fully understood, but it may be due to interactions between genes, nutrition and hormones.

Alcohol

Your risk of developing breast cancer can increase with the amount of alcohol you drink. Research shows that for every 200 women who regularly have two alcoholic drinks a day, there are three more women with breast cancer, compared with women who don't drink at all.

Radiation

Certain medical procedures that use radiation, such as X-rays and computerised tomography (CT) scans, may slightly increase your risk of developing breast cancer.

If you had radiotherapy to your chest area for Hodgkin lymphoma when you were a child, you should have already received a written invitation from the Department of Health for a consultation with a specialist to discuss your increased risk of developing breast cancer. See your GP if you weren't contacted, or if you didn't attend a consultation.

If you currently need radiotherapy for Hodgkin lymphoma, your specialist should discuss the risk of breast cancer before your treatment begins.

Hormone replacement therapy (HRT)

Hormone replacement therapy (HRT) is associated with a slightly increased risk of developing breast cancer. Both combined HRT and oestrogen-only HRT can increase your risk of developing breast cancer, although the risk is slightly higher if you take combined HRT.

It's estimated that there will be an extra 19 cases of breast cancer for every 1,000 women taking combined HRT for 10 years. The risk continues to increase slightly the longer you take HRT, but returns to normal once you stop taking it.

Want to know more?

• Breakthrough Breast Cancer: Breast cancer risk factors
• Breast Cancer Care: Am I at Risk of Developing Breast Cancer?
• Cancer Research UK: Breast cancer risks and causes
• Live Well: Breast cancer genes
• Macmillan: Breast cancer causes and risk factors