Possible breast cancer risks

What you eat may increase your breast cancer risk. Recent research indicates that saturated fat in the diet may play a small part in breast cancer risk. There is more about this in the 'foods we all eat' section of diet and preventing breast cancer.

A recent study showed that women who have one breast larger than the other may be at a slightly increased risk of developing breast cancer. But any increase in risk is small compared to other risk factors. For most women it is normal to have slightly uneven (asymmetrical) breasts. More research needs to be done before we know for sure how this may affect breast cancer risk. The important thing is to check your breasts regularly and be aware of how they usually look and feel. And to let your doctor know if you notice any changes, including unevenness.

Some women wonder if a past injury could have caused their breast cancer. Scientific research has not identified this as a risk factor but one study suggested a link. We need more studies to prove or disprove this. There is more information about the injury and breast cancer study in our breast cancer questions section.

A meta analysis of 20 individual studies reported that women with diabetes have a small increase in their risk of breast cancer. This may be due to higher levels of insulin during the initial phase of diabetes. But people with diabetes often have a higher body mass index (BMI) and a high BMI increases the risk of breast cancer for some people. So it is not clear why women with diabetes may have an increased risk.

A number of studies seem to show that night shift work can increase breast cancer risk. Some studies also show a higher risk of breast cancer in women who sleep for fewer hours at night. But this has not been proven for sure. At the moment we don't know how other lifestyle factors, such as taking hormone replacement therapy (HRT), being overweight, having fewer children and drinking alcohol, interact with shift work to increase a woman’s risk of breast cancer. We also don't know how many years of shift work might cause a significant increase in risk. But a large study by the Health and Safety Executive is due to end by 2011 and may give more information about the risks to British women.

Diethylstilboestrol (DES) is a drug that is no longer used in medicine. It was used between 1945 and 1970 as a treatment for pregnant women, to stop them having a miscarriage. Researchers are still gathering information about its effects. Some research studies have shown that women who took DES during pregnancy have a slightly increased risk of breast cancer. Researchers have also looked at the risk of breast cancer in women whose mothers took DES during their pregnancy but a summary of the published studies found no increase in risk.

Some people think that pesticides increase the risk of breast cancer. But there is no convincing scientific evidence for the role of any pollutants in breast cancer risk, including pesticides. If pollutants were a major risk factor, this would be very likely to have shown up in the research that has been done so far. The fact that there is no clear evidence shows that if pollutants do have any role in causing breast cancer, it is only likely to be a very small one.

There has been quite a bit of research into whether smoking causes breast cancer. Some large studies have shown that smoking can increase the risk of developing breast cancer. This increase has been shown for women who start smoking before the age of 20 or before having children. There is some disagreement between studies in this area, particularly about whether smoking increases the risk of breast cancer only before the menopause, or whether it affects risk for all women. More research is needed before we can be sure about this link.

Pregnancy termination (abortion) has been researched  looked into in several different studies. It doesn't seem to increase breast cancer risk. Researchers thought it might, because of the effect of pregnancy on breast cells.

Research has been done into the effects of stress on breast cancer. Cancer Research UK's London Psychosocial Group led a study of women with breast cancer and found that stress did not make breast cancer more likely to come back. Another study called the Nurses' Health Study found no link with breast cancer risk.