Causes & Risk Factors

Cause and Risk Factors of Breast Cancer Prevent Breast Cancer Charity UK

Understanding causes and risk factors are important steps towards the prediction and prevention of breast cancer.

Click on each individual risk factor below to find out more.

Age at 1st Menstrual Period (Menarche)

The younger a woman starts having periods the higher your risk of breast cancer in later life. The average age of menarche has been falling steadily in developed countries over the last century, probably as a result of better nutrition.The relative risk for breast cancer is reduced approximately by 7% for each year that menarche is delayed after 12 years of age.

Age at First Birth

The younger you are when you start child bearing, the lower your risk of breast cancer. This may be one of the factors that means breast cancer has been steadily rising in instance in Western countries over the last few decades, because the average age of having your first baby has steadily risen over this time.

The extra risk is estimated to be around 3% relative increase in risk for each year of delay. For example, a woman who has her first baby at age 30 would have a 15% additional theoretical risk compared to a woman who had her first baby at 25, all other factors being equal.

Age at Menopause

The younger your age of menopause, the lower your relative risk of breast cancer. This effect is seen regardless of whether you have had a natural menopause or one induced by surgery. If you have an early menopause and use HRT up until the age of 50, your risk of breast cancer is the same as someone who had a natural menopause at 50.


Women who drink more alcohol have higher rates of breast cancer and women who are tee-total have a lower risk of breast cancer compared to those who drink alcohol. Risk of breast cancer rises with our alcohol intake, and no particular level of alcohol intake is completely risk free.

Each extra alcoholic drink women have on a daily basis increases risk by 10%.

The extra risk is small if you are drinking sensibly and within government guidelines of 1– 2 units / day. If you drink one bottle of wine per day, your risk is likely to be 50% % higher than someone who does not drink at all. For women concerned about breast cancer risk, it therefore makes sense to drink less but not necessarily teetotal

Try to keep to sensible intakes of less than 14 units week. One unit of alcohol is equal to; 90 ml of wine i.e. half a 175 ml glass, 1/2 a pint of standard strength beer or larger, or a single measure (25ml) of any spirit.

Breast Density

The density of breast tissue can be calculated by looking at a standard mammogram. Women with denser breasts have a higher risk of breast cancer compared to those with less dense breast tissue. Part of this increased density is related to genetic factors and part to hormonal factors (such as menopausal status or being on HRT).

Breast Implants

Studies of women who have had silicone breast implants show that they have no increased risk of breast cancer. They do however present a small problem for women who are having regular screening mammogram, because they lower the accuracy of that test. Perhaps about 10% of breast tissue is not seen as clearly on a mammogram in someone with breast implants.

On the other hand, this negative effect may be compensated for by the fact that women with implants tend to have a smaller overall natural breast volume, which is displaced forward making breast examination easier. At a practical level, this theoretical problem should not deter someone from undergoing breast reconstruction surgery using an implant if, for example, they have had a mastectomy. It may be a consideration for those women with a strong family history who are contemplating cosmetic breast augmentation, and a reason not to proceed unless you are really sure of the benefit.


Breastfeeding appears to confer a degree of protection against breast cancer. The amount of benefit appears to be around 4% reduction in relative risk for every year that has been spent breastfeeding. Breastfeeding three children for 2 months each will thus have hardly any effect on your breast cancer risk, but breastfeeding for one year each would reduce your theoretical risk by 12%.

Contraceptive Pill

The contraceptive pill slightly increases your risk of breast cancer. The extra relative risk is about 20% in current users of the pill. However, because the pill tends to be used in young women whose background risk of breast cancer is very small, this amounts to only a very few extra cases of breast cancer.

The risk associated with the pill falls once you have stopped taking the pill, and falls back towards normal after 5 years or more. For women who are concerned about their breast cancer risk, it is sensible to plan to stop using oral contraceptives at around 30 years of age. Below this age, the extra risk is very small indeed. However, your background risk of breast cancer increases as you get older and so the additional risk of using the pill becomes relatively higher.

Environmental Chemicals

Studies looking at environmental chemicals such as pesticides, fertilisers and preservatives have as yet not shown any direct link with breast cancer. One example is the group of preservatives known as parabens, which are found in a number of skin creams, deodorants and personal care products.

Research funded by Prevent Breast Cancer has not shown any direct link between chemicals such as deodorants and breast cancer. Nevertheless, such chemicals can be found in breast tissue and indeed may accumulate in this tissue. The level of risk associated with these chemicals is unknown.

Exercise and Physical Activity

Women who take regular exercise have lower rates of breast cancer. Quite how much exercise that helps prevents the development and growth of breast cancer is not known. Being active helps maintain a healthy body weight which reduces risk of breast cancer. Exercise also lower levels of hormones in the body which may in turn limit the growth of breast cancer cells

Try to do at least 30 minutes of moderate exercise (activity which makes your hard beat a bit faster and makes you feel slightly out of breath) five times or more a week. The more active you are, the more you can reduce your risk. Exercise seems to be most protective in women who are also a healthy weight.

Family History

Women with a family history of breast cancer have a higher risk of developing the disease. Thus, if your mother or sister has had breast cancer, your relative risk may be double that compared to a person with no family history. However, 85% of women with an affected first degree relative (mother or sister) will never develop the disease and over 85% of breast cancer patients have no family history at all. In a small number of women with a particularly strong family history, the risk of breast cancer may be caused by a mutation in a BRCA1 or BRCA2 gene.

If you come from a family where breast cancer has affected several family members, then you should ask your GP for a referral to a genetic clinic where gene testing can be provided on the NHS. Recent research suggests that, even in the 85% of women with no family history at all, there may nevertheless be an underlying genetic predisposition caused by tiny faults or variations in genetic make-up known as ‘SNPs’.

Women who carry several such SNPs may have a much higher chance of breast cancer than those who carry none, and yet have no family history of the disease to indicate that such a genetic predisposition might be present. In the future, it may be that testing for SNPs will become a useful means of early detection of the disease.

Hodgkin’s Disease

If you are a woman who was treated for Hodgkin’s disease before you were 35 years of age, you may have received ionising radiation as part of your treatment. The cure rates following such treatment are extremely good. However, the subsequent lifetime risk of developing breast cancer in increased so that as many as 1:3 of such women may develop breast cancer. If you have had Hodgkin’s Disease you are therefore eligible to have special surveillance within the NHS.


HRT use also increases the risk of breast cancer for current users. This additional risk is seen particularly with oestrogen/progesterone combinations. Once HRT has been stopped, the risk reduces back down to the average risk over the following few years. The risk rises with the more years of HRT use. Nevertheless, HRT use can have many benefits and so this potential risk needs to be balanced against those potential benefits.

For women who have never used HRT, the risk of breast cancer is 45 per 1000 women between the ages of 50 and 70 years. The additional numbers of breast cancers diagnosed between these ages per 1000 women who began to use HRT at age 50 is an extra two cases (after 5 years use) 6 cases (after 10 years use) or 12 cases (15 years use).

As you can see, the level of risk is very small for women who have used HRT for only 5 years. The decision about HRT use therefore requires a careful balance of potential benefits and potential risks. As a general rule, women for whom breast cancer risk is a concern should limit their use of HRT to perhaps no more than about 5 years.

Number of Pregnancies

The more full time pregnancies you have had, the more your potential protection from breast cancer. Even in the absence of breastfeeding, there appears to be a relative reduction in risk by about 7% for each child birth compared to someone who has not had children (also known as ‘nulliparous’). Overall, there may be a 30% increase in risk comparing nulliparous women with those who have had children.

Personal History of Breast Cancer

If you have had breast cancer once in the past, your risk of developing a second primary breast cancer in the opposite breast is around 0.5% risk per annum, in other words, after 10 years, a 5% risk and after 20 years a 10% risk. This level of risk can be almost halved if you have taken Tamoxifen regularly for a 5 year period. Thus, after a 20 year period your risk would only be 5%. For a woman who has previously had breast cancer, it does therefore make sense to continue having regular mammograms throughout the rest of your lifetime.

Physical Activity

Another feature of western lifestyle is that we tend to take less physical activity compared to previous generations. There is now good evidence that physical activity has a protective effect against breast cancer. This may partly be through controlling body weight, but also appears to be due to a positive effect on body hormone levels. Building regular physical activity into your weekly routine therefore seems to be another practical way in which you can lower your individual breast cancer risk.

Previous Breast Lumps

The majority of lumps in the breast are harmless (benign) and carry no associated increased risk of breast cancer. Two examples of such lumps are fibroadenomas and breast cysts, both of which are completely harmless. Women who have multiple cysts over a period of several years do have a slightly increased risk of breast cancer. If you are in this category, it is sensible advice to continue having regular mammograms and check-ups until the tendency to cyst formation has settled down.

There are also some more unusual types of breast lump that do have an associated increased risk of breast cancer. These are known as ‘proliferative’ breast disease or ‘atypical hyperplasia’. If you have had such a lump, your risk of breast cancer may be between two to five times greater than the average risk, and so regular mammogram surveillance is recommended.


Some research suggests that smoking increases the risk of breast cancer. Risk appears to be greater if you started smoking at a younger age, particularly if you started smoking before you started having children. Risk appears to be greater for heavy lifetime smokers. It is not known whether passive smoking increases risk.

Smoking is bad for health and a major cause of lung and other cancers, heart disease , stroke, and diabetes. For advice on stopping smoking, contact your GP or find your local NHS Stop Smoking Service through the website or call the free phone Smoke free National Helpline to speak to a trained adviser on 0800 0224 332.


Being overweight and gaining weight over adult life can increase the risk of developing breast cancer. The average woman in the UK gains a stone in weight over adult life. This increases risk of breast cancer by about 20% whilst gaining 1 and ½ stone increases risk by 60% and 3 stone doubles risk. It makes sense to try and limit weight gain as we age. If you are someone who has already gained weight, you will be interested to know that weight loss seems to help reduce this extra risk. We were one of the first research groups to find that weight loss can reduce the risk of breast cancer. In our study of 34,000 women modest weight loss i.e. losing 10 pounds of weight reduced risk by 25-40%.

Western Diet

As countries become more westernised, so their risk of breast cancer increases. Some of the most dramatic rises in breast cancer are now being seen in such countries like Japan, China and India. Women who migrate from a country with low breast cancer rates into a western country often experience a dramatic rise in their risk. One of the reasons for this appears to be western diet. The main culprit is not any particular food item or additive but the high fat and sugar content, the availability of fast foods and snacks, hence the overall high calorie content of a typical western diet. A number of studies have looked for any links between certain foods and breast cancer such as saturated fat, milk, soya, fibre vitamins, antioxidants, food additives. These studies suggest that an overall healthy diet is most important, as well as being a healthy weight, taking regular exercise and moderating alcohol

We recommend a well-balanced Mediterranean diet to maintain a healthy weight. This type of diet also reduces the risk of heart disease, stroke, dementia, diabetes, and other cancers.

A Mediterranean diet is one which is;

  • High in mono-unsaturated fat (avocados, nuts, olives, olive oil and rapeseed oil) and low in saturated fat (fatty meats, full fat dairy products, butter, palm oil, coconut oil, many shop bought cakes, biscuits and pastry)
  • Includes a variety of vegetables (aim for 5 portions of vegetables a day) and some fruit.
  • Includes some lean protein such as lean meats, chicken, fish ,tofu, beans and lentils
  • Includes oily fish such as salmon, trout and mackerel (2 portions per week)
  • Includes wholegrain carbohydrates such as wholemeal bread, brown rice and pasta, potatoes with skins.
  • Includes some low fat dairy products (aim for 3 portions a day) such as semi-skimmed or skimmed milk, low fat yoghurts, and low fat cheese.
  • Limits added sugar in the diet, e.g. cakes, biscuits, some cereals, sugary drinks, sugar, syrups and honey.


The risk of cancer is increased in people who have had an excessive exposure to ionising radiation. This leads to an understandable concern that too many mammograms could in themselves increase the risk of breast cancer. Thankfully, the dosage of x-rays received in a mammogram is low. It has been estimated that an individual would need to undergo 25,000 mammograms to produce a breast cancer.

Studies looking at the UK population as a whole, taking into account all diagnostic x-rays and scans as well as mammograms, suggests that 1:1000 breast cancers may be attributed to such diagnostic x-rays. Although this level of risk if extremely small, it is sensible advice to avoid having unnecessary x-rays and to avoid having regular mammograms until; if possible, you are over 35 years of age.