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Causes - Breast cancer in women

We are still learning about the causes of breast cancer. It is not always clear why breast cancer happens.

Certain things can affect your risk of getting breast cancer. These are called risk factors.

They include:

  • age
  • carrying extra weight
  • drinking alcohol or smoking
  • if you had cancer or certain types of breast lumps before
  • family history
  • breast density
  • hormones and hormone medicine
  • certain types of radiation

Some of these you cannot do anything about.

But there are some that you can. For example, by keeping a healthy weight, drinking less or no alcohol, getting help to quit smoking.

Lifestyle changes to lower your risk of breast cancer

Non-urgent advice: Contact your GP if you have any:

Tell your GP about any risk factors, such as family history of cancer.


The risk of developing breast cancer increases with age. The condition is most common in women age 50 or older who have been through menopause.

About 8 out of 10 cases of breast cancer cases happen in women over 50.

BreastCheck is the breast cancer screening programme.

If you are age 50 to 69 you'll be offered breast screening every 2 years. Due to COVID-19, invitations for screening have been delayed by up to 1 year.

If you had breast cancer before

You have a higher risk of developing breast cancer again if you've had it before.

The risk is also higher if you've had early non-invasive cancer cell changes in your breast ducts.

Previous breast lump

You may have had a previous lump which was breast cancer or a benign lump.

A benign (non-cancerous) breast lump does not mean you have breast cancer.

But certain types of breast lumps may slightly increase your risk of developing cancer.

These include:

  • cells growing abnormally in ducts (atypical ductal hyperplasia)
  • abnormal cells inside your breast lobules (lobular carcinoma in situ)

A breast lobule is a gland that makes milk.

Family history

If you have close relatives who have had breast cancer, you may have a higher risk of developing breast cancer. A close relative is a mother, sister, grandmother or aunt.

Your risk increases if:

  • you have more than 1 relative who developed breast cancer
  • your relative developed breast cancer before the age of 50
  • you have gene changes linked to breast cancer

Gene changes linked to breast cancer

BRCA1 and BRCA2 are gene changes that affect how well your body can fix damage in your cells.

Women with this gene have a 6 in 10 to an 8 in 10 chance of developing breast cancer by age 80. These genes can be passed on from a parent to their child.

There are other genes that are linked to breast cancer.

What you can do if you have a family history of breast cancer

Non-urgent advice: Contact your GP if:

  • breast or ovarian cancer runs in your family and you're worried you may get it too

They may refer you to a breast cancer clinic or to a genetic service.

If they refer you for a genetic test, the results will show you if you have inherited 1 of the cancer-risk genes.

Dense breasts

Your breasts are made up of different types of breast tissue. If you have a higher amount (density) of a type of breast tissue called glandular tissue, you have a higher risk of getting breast cancer.

This may be because there are more cells that can become cancerous.

Glandular tissue is made up of thousands of tiny glands (lobules) that produce milk. It is non-fatty tissue and is denser than other types of breast tissue such as fatty tissue.


Not all women with dense breasts will get breast cancer.

Who is more likely to have dense breasts

The density of your breasts can change over time.

When you are younger you tend to have denser breasts. This is because you have more glandular tissue in your breast than fatty tissue.

As you get older, the amount of glandular tissue in your breasts decreases. It is replaced by fat, so your breasts become less dense.

You're more likely to have dense breasts if you:

How to find out if you have dense breasts

You cannot tell if you have dense breasts by look or feel.

A doctor can tell if you have a high breast density by looking at a mammogram - dense breast tissue looks solid and white on a mammogram. But it is hard to measure exactly how dense your breasts are.

If you find out you have dense breasts it does not mean you are less likely to survive breast cancer if you get it.

But having dense breasts makes it harder to spot cancer on a mammogram.

Mammograms and breast density

Hormones and hormone medicine

Some hormones and hormone medicine affect the risk of breast cancer.


There is an increased risk of developing breast cancer if you are taking combined HRT (oestrogen and progestogen). But the risk is very low.

Ask your GP about the risks and benefits of HRT before you start taking it. For some people, the benefits of HRT outweigh this very low risk. Your GP can help you decide.

HRT and breast cancer risk

Contraceptive pill

Women who use the contraceptive pill have a slightly increased risk of developing breast cancer.

The risk starts to decrease when you stop taking the pill. Your risk of breast cancer goes back to normal 10 years after stopping.

Exposure to oestrogen

The female hormone oestrogen can sometimes stimulate breast cancer cells and cause them to grow.

Your eggs are stored in your ovaries. Your ovaries begin to produce oestrogen when you start puberty. This is to regulate your periods.

The more oestrogen you are exposed to, the more it affects your breast cells. This can slightly increase your risk of breast cancer.

How much oestrogen you are exposed to depends on many things.

You may be exposed to more oestrogen if you:

  • have periods before age 12 or after 50
  • never get pregnant
  • have your first pregnancy after 30
  • carrying extra weight after menopause - this can cause your body to produce more oestrogen

For example, you might start having periods at a young age (before age 12) and experience the menopause later than average (menopause usually happens between age 45 and 55). This means you will have been exposed to oestrogen over a longer period of time.

In the same way, your risk of developing breast cancer increases if you have never been pregnant or do not have a baby until later in life. This is because your exposure to oestrogen is not interrupted by pregnancy.

Lifestyle risk factors

Some lifestyle factors may increase your risk of breast cancer. These are factors you can do something about.

Carrying extra weight

Your risk of developing breast cancer after menopause increases if you have a body mass index (BMI) of 30 or over.

This is thought to be linked to the amount of oestrogen in your body.

If you have gone through menopause and are overweight or have obesity, you may be more at risk of developing breast cancer.

Lifestyle changes if you have obesity


You have a greater risk of breast cancer if you smoke or you smoked regularly in the past.

Tobacco products, including cigarettes, have chemicals in them that cause cancer.

Get help to quit smoking


Your risk of developing breast cancer increases with the amount of alcohol you drink.

Alcohol can also increase the levels of some hormones, such as oestrogen. This can increase the risk of breast cancer.

Weekly low-risk alcohol guidelines


You may have an increased risk of breast cancer if you had radiotherapy to the chest area for Hodgkin’s lymphoma.

Radiation and your mammogram

Content supplied by the NHS and adapted for Ireland by the HSE

Page last reviewed: 11 October 2023
Next review due: 11 October 2026

This project has received funding from the Government of Ireland’s Sláintecare Integration Fund 2019 under Grant Agreement Number 123.