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Causes and risk factors - Breast cancer in women

The causes of breast cancer are not fully understood. It is difficult to say why one woman may develop breast cancer and another may not.

Certain risk factors can affect how likely you are to develop breast cancer. Some of these you cannot do anything about.

But there are some risk factors related to your lifestyle that you can change. For example, if you drink alcohol or smoke.

Non-urgent advice: Talk to your GP if you have any:

Make your GP aware of any risk factors, such as your age or any family history of cancer.


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

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

All women aged 50 to 69 should be screened for breast cancer every 2 years as part of the Breast Screening Programme - BreastCheck. Because of COVID-19 there is a 1-year delay in screening. Women are being screened every 3 years. But this is only temporary.

If you've had breast cancer

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. This could have been in either breast.

Previous breast lump

A benign breast lump does not mean you have breast cancer.

Certain changes in your breast tissue can make getting breast cancer more likely.

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 one relative who developed breast cancer
  • your relative developed breast cancer before the age of 50
  • you have the BRCA1 and BRCA2 gene changes

BRCA1 and BRCA2 are gene changes that affect you body's ability to fix damaged cells.

Women with this gene have a 45% to 65% chance of developing breast cancer by the age of 70. It's possible for these genes to be passed on from a parent to their child.

If you're worried about your family history of breast cancer, discuss this with your GP.

Breast density

Breast density is how breast tissue looks on a mammogram.

Your breasts are made up of 2 types of tissue - fatty tissue and non-fatty tissue. Dense breasts are breasts that have a high amount of non-fatty tissue.

Not all women with dense breasts will get breast cancer. But women with dense breasts have a higher risk. We don't know why this is. But the more dense your breasts are, the higher your risk.

Having dense breasts does not mean you are less likely to survive breast cancer if you get it.

Who is more likely to have dense breasts

You’re usually more likely to have dense breasts if you:

The density of your breasts can change over time. Younger women tend to have denser breasts. As you get older, your breasts become less dense.

How to tell if you have dense breasts

You cannot tell dense breasts by look or feel. But a doctor can tell if you have dense breasts by looking at a mammogram.

It is hard to measure exactly how dense your breasts are. This is because there's no agreed way of measuring breast density in Ireland.

Talk to your GP if you think you need a mammogram to check if you have dense breasts. They will also think about other factors, like your age and any family history of cancer.

Screening and breast density

Screening is not routinely performed in women under 50 years.

Dense breast tissue (non-fatty tissue) looks solid and white on a mammogram (X-ray). You cannot see through it. This makes the mammogram more difficult to read.

It means lumps or areas of abnormal tissue are harder to spot. This is why screening using mammograms is less effective for women with dense breasts.

Hormones and hormone medicine

Hormone replacement therapy (HRT)

There is an increased risk of developing breast cancer if you are on hormone replacement therapy (HRT). But the risk is very low.

Talk to 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.

Contraceptive pill

Women who use the contraceptive pill have a slightly increased risk of developing breast cancer. The risk starts to decrease once you stop taking the pill.

Your risk of breast cancer goes back to normal 10 years after stopping the pill.

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. This is 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, 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, not having children or having children later in life may slightly increase your risk of developing breast cancer. This is because your exposure to oestrogen is not interrupted by pregnancy.

Lifestyle risk factors

Some risk factors of breast cancer you cannot change, such as your age or breast density. But there are some lifestyle decisions that may increase your risk. These are factors you can do something about.


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.

Quit smoking, if you smoke


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, which can increase the risk of breast cancer.

Weekly low-risk alcohol guidelines

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. Being overweight or obese after the menopause causes more oestrogen to be produced.

You can reduce your risk by keeping a healthy body weight and not putting any weight you lose back on.


You may have an increased risk of breast cancer if you have a history of Hodgkin’s Lymphoma and received radiotherapy to the chest area.

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

Page last reviewed: 20 May 2022
Next review due: 20 May 2025

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