Cancer is caused by the cells in a certain area of your body dividing and multiplying too rapidly. It is not fully understood why breast cancer occurs, but research into the causes of breast cancer is continuing.

So far, several likely causes of breast cancer have been identified, as well as risk factors that may make developing the condition more likely. The various causes and risk factors are outlined below.

Causes and risk factors of breast cancer

Age

Your risk of developing breast cancer increases as you get older. Breast cancer is most common among postmenopausal women who are over the age of 50.

All women between 50-70 years of age should be screened for breast cancer every year.

Family history

The majority of breast cancer cases are not hereditary (run in families), although having breast cancer in your family can increase your chances of developing the condition. Particular genes, known as BRCA1 and BRCA2, can increase your risk of developing both breast and ovarian cancer, and it is possible for them to be passed on from one family member to another. There is also a third gene associated with an increased risk of breast cancer, which is known as TP53.

Therefore, you have a higher than average risk of developing breast cancer if you have close relatives who have had cancer of the breast, or ovary. However, as breast cancer is the most common form of cancer in women, it is also possible for it to occur more than once in the same family by chance.

If you have two, or more, close relatives from the same side of your family, such as your mother, sister, or daughter, who have had breast cancer, you should be eligible to be screened for breast cancer, or for the genes that may make developing it more likely.

Previous diagnosis of breast cancer

If you have had breast cancer before, you are more at risk of developing the condition in your other breast. If this is the case, you should be closely monitored by your treatment team, such as your oncologist (specialist in cancer) and your breast nurse, so that any recurring cancer can be detected as soon as possible.

Previous benign breast lump

While having a benign breast lump does not mean that you have breast cancer, it can slightly increase your risk of developing it. Certain changes in your breast tissue, such as atypical hyperplasia (cells growing abnormally), or lobular carcinoma in situ (benign cell changes inside your breast lobes) can also make getting breast cancer more likely.

Research shows that having atypical hyperplasia can increase your risk of developing breast cancer by 2-5 times. If you have either of these conditions, your doctor should monitor the health of your breasts with regular check ups.

You should always visit your doctor if you notice any lumps, or changes, in the appearance of your breasts.

Breast density

Your breasts are made up of thousands of tiny glands, or lobules, which produce milk, as well as fat and connective tissue. The glandular tissue contains a higher concentration of breast cells than the fat, or connective tissue, which makes it denser. Therefore, having dense breast tissue can increase your risk of developing breast cancer because there are more cells that can become cancerous.

Dense breast tissue can also make a breast scan (mammogram) harder to read because it makes any lumps, or areas of abnormal tissue, harder to spot.

Despite the fact that the risk of breast cancer increases with age, it is younger women who tend to have denser breasts. As you get older, the amount of glandular tissue in your breasts decreases and is replaced by fat, and your breasts become less dense.

Exposure to estrogen

In some cases, breast cancer cells can be stimulated by the female hormone estrogen. Your ovaries (where your eggs are stored) begin to produce estrogen when you enter puberty in order to regulate your periods.

Your risk of developing breast cancer may rise slightly with the amount of estrogen that your body is exposed to. For example, if you started your periods at a young age, and entered menopause at a late age, you will have been exposed to estrogen 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 estrogen is uninterrupted by pregnancy.

Being overweight or obese

If you are postmenopausal, and you are overweight, or obese, you may be more at risk of developing breast cancer. This is thought to be linked to the amount of estrogen in your body, as being overweight, or obese, causes more estrogen to be produced.

You can find out if you are overweight, or obese, by using the body mass index (BMI) calculator.

Being tall

If you are taller than average height, you are more likely to develop breast cancer than those who are shorter than average. The reason for this is not fully understood, although it may be because taller women tend to have more breast tissue than women who are of average height, or less.

Alcohol

Your risk of developing breast cancer can increase with the amount of alcohol that you drink. Research shows that there will be three extra women diagnosed with breast cancer for every 200 women who regularly have two alcoholic drinks a day, when compared with women who do not drink at all.

Radiation

Certain medical procedures which use radiation, such as X-rays and CT scans, may slightly increase your risk of developing breast cancer. However, the amount of radiation that is used during these procedures is always the lowest possible, and they are only carried out when they are medically necessary.

If you had radiotherapy for Hodgkin's lymphoma when you were a child, or you currently need radiotherapy for Hodgkin's 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 estrogen-only HRT can increase your risk of developing breast cancer, although the risk is slightly higher again if you take combined HRT.

The research behind this is still in its early stages, but is estimated that there will be an extra 19 cases of breast cancer for every 1,000 women who are taking combined HRT for ten years. The risk continues to increase slightly the longer you take HRT, but returns to normal once you stop taking it.