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Risks and causes

Find out about the causes of breast cancer, including lifestyle factors, hormone levels, and other medical conditions.

Breast cancer is now the most common cancer in the UK (excluding non melanoma skin cancer). It is by far the most common cancer in women.

1 in 8 women in the UK develop breast cancer during their lifetime.

Anything that increases your risk of getting a disease is called a risk factor.

Having one or more of the following risk factors doesn’t mean that you will definitely get cancer. Many people who have them never get it and some people with no risk factors develop it.

Risks and causes

Like most cancers, the risk of breast cancer increases as you get older. 

Most women who get breast cancer have had their menopause. Around 2 out of every 10 women (20%) are under 50 years old.

It is important to have mammograms as part of the national breast screening programme between the ages of 47 to 73.

Drinking alcohol increases your risk of breast cancer by a small amount. Your risk increases with every extra unit of alcohol you have per day. One unit is a half pint of beer, a small glass of wine, or a measure of spirits.

Latest UK government guidelines advise drinking no more than 14 units of alcohol a week.

Women who are overweight after their menopause have a higher risk of breast cancer than women who are not overweight. Men also have an increased risk of breast cancer if they are overweight or obese.

Being overweight means having a body mass index (BMI) of between 25 and 30. Obesity means being very overweight with a BMI of 30 or higher.

Try to keep a healthy weight by being physically active and eating a healthy, balanced diet.

Women who are taller than average have a slightly increased risk of breast cancer after the menopause.

This could be due to different hormone levels in taller women.

Some people have a higher risk of developing breast cancer than the general population because other members of their family have had particular cancers. This is called a family history of cancer. 

Having a mother, sister or daughter diagnosed with breast cancer approximately doubles the risk of breast cancer. This risk is higher when more close relatives have breast cancer, or if they are under 50. But more than 8 out of 10 women who have a close relative with breast cancer will never develop it.

UK guidelines help GPs identify people who might have an increased risk of cancer due to their family history.

The guidelines recommend that your GP should refer you to a specialist breast clinic for assessment if you have any of the following:

  • One first degree female relative diagnosed with breast cancer aged younger than 40 (a first degree relative is your parent, brother or sister, or your child)
  • One first degree male relative diagnosed with breast cancer at any age
  • One first degree relative with cancer in both breasts where the first cancer was diagnosed aged younger than 50
  • Two first degree relatives, or one first degree and one second degree relative, diagnosed with breast cancer at any age (second degree relatives are aunts, uncles, nephews, nieces, grandparents, and grandchildren)
  • One first degree or second degree relative diagnosed with breast cancer at any age and one first degree or second degree relative diagnosed with ovarian cancer at any age (one of these should be a first degree relative)
  • Three first degree or second degree relatives diagnosed with breast cancer at any age

Your GP should also refer you if you have one first degree or second degree relative diagnosed with breast cancer when they were older than 40 years and one of the following:

  • The cancer was in both breasts (bilateral)
  • The cancer was in a man
  • Ovarian cancer
  • Jewish ancestry
  • Sarcoma (cancer of the bone or soft tissue) in a relative younger than age 45 years
  • A type of brain tumour called glioma or childhood adrenal cortical carcinomas
  • Complicated patterns of multiple cancers diagnosed at a young age
  • Two or more relatives with breast cancer on your father's side of the family

The specialist breast clinic staff can work out your risk of developing breast cancer and advise you whether you might need extra screening.

A very strong family history of breast cancer might mean there is a faulty gene in your family. There are several gene faults that can increase breast cancer risk.

Having one of these faulty genes means that you are more likely to get breast cancer than someone who doesn’t. But it is not a certainty.

Remember that most breast cancers happen by chance. Only about 3 out of every hundred (3%) are related to a known inherited breast cancer gene.

Having had breast cancer increases your risk of getting another breast cancer. It might occur in the same breast or in the other breast.

Your specialist will keep a close eye on you with regular check ups. So, a new cancer should be picked up early.  

Having other types of cancer can also increase your risk.

People who had radiotherapy to the chest for Hodgkin lymphoma when they were young have a higher breast cancer risk.

Breast cancer risk is also higher in people who have had any of the following:

  • melanoma skin cancer
  • lung cancer
  • bowel cancer
  • womb cancer
  • a type of leukaemia called chronic lymphocytic leukaemia

A large report found that the risk of breast cancer is higher in white women than any other ethnic group. This is at least partly due to lifestyle factors.

Levels of the female sex hormone, oestrogen, and the male hormone, testosterone, can affect the risk of breast cancer. Women have small amounts of the male hormone testosterone in their bodies.

After the menopause, women with higher levels of oestrogen and testosterone in their blood have a higher risk of breast cancer compared to women with the lowest levels. Women with higher levels of testosterone in their blood before menopause have a higher risk of breast cancer.

There is an increased risk of breast cancer in women with higher levels of a hormone called insulin like growth factor 1 (IGF-1). It is not clear what controls levels of IGF-1 in the bloodstream. It is probably related to our genes, body weight, and how much exercise we do.

You have an increased risk of breast cancer if your periods started early (before the age of 12). If you have a late menopause (after the age of 55) this increases your breast cancer risk compared to women who have an earlier menopause. This may be linked to hormone levels.

Many women take hormone replacement therapy (HRT) to reduce menopausal symptoms. There are 2 main types of HRT – combined HRT (oestrogen and progesterone) and oestrogen only HRT. 

HRT increases the risk of breast cancer while women take it and for up to 5 years afterwards. Combined HRT (oestrogen and progesterone) is more likely to cause breast cancer than oestrogen only HRT.

HRT can have some health benefits and so if you are worried about taking it, talk to your doctor about the benefits and risks in your individual situation.

There is a very small increased risk of breast cancer when you take the contraceptive pill. This increase in risk goes back to normal 10 years after you stop taking it.

Remember that breast cancer is rare in young women. Most women who take the pill are in their late teens, twenties and early thirties. So a small increase in this risk during the time women take the pill means very few extra cases of breast cancer.

DCIS and LCIS are changes within the breast tissue that might develop into breast cancer in some women. DCIS stands for ductal carcinoma in situ. LCIS stands for lobular carcinoma in situ.

Women with a diagnosis of DCIS or LCIS have double the usual risk of invasive breast cancer in the same or other breast. About 1 in 6 women with a previous carcinoma in situ of the breast have a risk of developing breast cancer within 10 years.

It is important to remember that most women with LCIS or DCIS will not develop invasive cancer.

Benign breast disease means non cancerous breast conditions.

There are 3 types:

  • non proliferative
  • proliferative without atypia
  • proliferative with atypia (atypical hyperplasia)

Breast disease that is not growing and where the cells are not dividing is called non proliferative and doesn’t usually increase the risk of breast cancer. But if you have a strong family history of breast cancer you might have a small increased risk.

Breast lumps with an overgrowth of cells (proliferation) but without abnormal (atypical) cells increase the risk of breast cancer compared to the average risk.

About 1 in 20 breast lumps (5%) show atypical hyperplasia. This means the cells are not cancer, but are growing abnormally. Atypical hyperplasia increases your risk of breast cancer by about 3 times the average. Atypical hyperplasia is uncommon and if you haven't been told that your breast lump showed these changes, it probably didn't. But if you are worried, you can ask your doctor about it.

You should always get breast lumps checked out straight away to make sure they are not cancer.

Breast cancer risk is higher in women with the most dense breast tissue compared to less dense tissue. Women with dense breast tissue have less fat and more breast cells and connective tissue in their breasts.

Our genetic make up affects breast density.

X-rays

Exposure to radiation is known to increase the risk of many types of cancer. Most of us are never exposed to enough radiation to make much difference to our risk.

Nowadays, doctors keep medical exposure to radiation as low as possible. They don't do x-rays or CT scans unless they really need to. And the amount of radiation used is very small.

Many women worry about having mammograms as part of breast screening as it exposes them to x-rays. But the amount of radiation you have with a mammogram is very small.

Radiotherapy

Radiotherapy treatment for breast cancer increases the risk of getting breast cancer in the other breast by a small amount. But this small risk is balanced by the need to treat the original breast cancer.

If you had radiotherapy to your chest for Hodgkin lymphoma in the past, you could be at increased risk of getting breast cancer. If you need radiotherapy for Hodgkin lymphoma your doctors will tell you about this risk. They will offer you breast screening if it is appropriate.

It is important to remember that second cancers are usually found early when they can be successfully treated. Also, radiotherapy treatments are now more focused than in the past.

Women with diabetes have a small increase in their risk of breast cancer, although we are not sure why.

Breast cancer risk is increased by about 3 times in women who have autoimmune thyroiditis (a condition that causes damage to the thyroid gland). An overactive or underactive thyroid doesn’t increase your risk.

High bone mineral density is linked to an increased risk in breast cancer in women after their menopause. This could be due to high levels of oestrogen in patients with high bone mineral density.

Women who have children have a slightly lower risk of breast cancer than women who don't have children. The risk reduces further the more children you have.

Your age when you have your first child also has an effect. The younger you are when you have your first child, the lower your risk.

Possible breast cancer risk factors

Some factors might increase the risk of breast cancer but there is not enough evidence to be sure.

Smoking

Smoking tobacco might increase your risk of getting breast cancer. It is never too late to stop smoking but the sooner you stop the better.

Night shifts

Some research has shown that working night shifts could possibly slightly increase breast cancer risk. This may be because broken or shorter periods of sleep lowers the level of a hormone called melatonin.

Factors that reduce the risk of breast cancer

Some factors can reduce the risk of breast cancer

Cancer controversies

Stories about potential causes of breast cancer are often in the media and it isn’t always clear which ideas are supported by evidence. There might be things you have heard of that we haven’t included here. This is because either there is no evidence about them or it is less clear.

Theories about causes of breast cancer in the media have included:

  • uneven breasts
  • breast injury
  • bras
  • antibiotics
  • terminating a pregnancy

More information about breast cancer risks and causes

Last reviewed: 
23 Aug 2016
  • Familial breast cancer: classification, care and managing breast cancer and related risks in people with a family history of breast cancer
    The National Institute for Health and Care Excellence, August 2015

  • Cancers attributable to overweight and obesity in the UK in 2010
    Parkin DM and Boyd L
    Br J Cancer. 2011 Dec 6;105 Suppl 2:S34-7

  • Cancers in 2010 attributable to ionising radiation exposure in the UK
    DM Parkin and SC Darby
    Br J Cancer, 2011 Dec; 105(Suppl 2): S57-S65

  • Breast cancer and hormone-replacement therapy in the Million Women Study
    V Beral and Million Women Study Collaborators
    Lancet Oncology, 2003 Aug; 362(9382): 419-427

  • Familial breast cancer: collaborative reanalysis of individual data from 52 epidemiological studies including 58,209 women with breast cancer and 101,986 women without the disease
    Collaborative Group on Hormonal Factors in Breast Cancer
    Lancet. 2001 Oct 27;358(9291):1389-99

  • The information on this page is based on literature searches and specialist checking. We used many references and there are too many to list here. If you need additional references for this information please contact patientinformation@cancer.org.uk with details of the particular risk or cause you are interested in.

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