Breast cancer risk factors and causes
The causes of breast cancer are unknown. But certain things called risk factors can affect a woman’s chances of getting breast cancer. Having risk factors doesn’t necessarily mean you will get breast cancer. Some women get it while others (with the same risk factors) don’t.
Even though the exact causes of breast cancer aren’t fully known, it’s likely to be caused by a combination of different risk factors rather than just one.
Here we explain some of the known breast cancer risk factors:
The risk of breast cancer increases with age. It’s rare in women under 35, and 8 out of 10 breast cancers (80%) occur in women aged 50 or over.
Previous cancer and other breast conditions
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Women who’ve had breast cancer or other breast conditions in the past may be at a higher risk of developing breast cancer. This includes women who have previously had:
Exposure to the hormones oestrogen and progesterone for long, uninterrupted periods can affect your breast cancer risk. Factors that increase this risk include:
taking combined hormone replacement therapy (HRT) containing oestrogen and progesterone over several years (if you’re over 50)
not having children or having them later in life
not having breastfed or breastfeeding for less than a year
starting your periods early (under 12) or having a late menopause (after 50)
taking the contraceptive pill (but the risk reduces if you stop taking it).
The following lifestyle factors may slightly increase your risk of breast cancer:
Drinking more than two units of alcohol a day over many years can damage your liver. This increases your breast cancer risk because the liver helps to control oestrogen levels.
After the menopause, body fat is the main source of oestrogen. So if you’re overweight, the level of oestrogen in your body may be high, increasing your breast cancer risk.
Smoking heavily over many years, especially if you started smoking at a young age, increases your risk.
Genetic factors (family history)
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Only 5–10% of breast cancers are thought to be linked to an inherited breast cancer gene. Our genes store the biological information we inherit from our parents. The genes most commonly linked to an increased risk of breast cancer in families are BRCA1 and BRCA2. Other genes have been identified, but they only slightly increase the risk. If you have one relative who was diagnosed with breast cancer at an older age, it’s not likely that the cancer is due to an inherited breast cancer gene.
A genetic change (mutation) that increases the risk of breast cancer is only likely to be present in a family if:
there are three close relatives on the same side of the family who developed breast cancer at any age
there are two close relatives on the same side of the family who developed breast cancer under the age of 60
there is one close relative who developed breast cancer under the age of 40
there is a close male relative with breast cancer
there is a close relative with breast cancer in both breasts
there is a close relative with breast cancer and another relative on the same side of the family with ovarian cancer.
Close relatives, sometimes called your first degree relatives, are parents, children, sisters and brothers.
Our section on cancer genetics explains more about family history and assessing risk.
Women aged 40–50 may be able to have regular screening before they reach 50, depending on their degree of risk.
We have more information about breast screening in women under 50 with a family history of breast cancer.
Online risk assessment
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We have an online interactive tool called OPERA (Online Personal Education and Risk Assessment), which gives personalised information about inherited breast cancer risk. OPERA is based on guidance on familial breast and ovarian cancer from the National Institute for Health and Clinical Excellence (NICE). NICE is an independent body that currently gives guidance on NHS services in England and Wales.
OPERA asks you a number of questions about your personal and family history of breast and ovarian cancer. It then gives a tailored personal assessment based on your answers.
The program also gives details of websites, helpful information and further support. OPERA isn’t supposed to take the place of professional genetic counselling services, so if you’re concerned about genetic risk, you should still consult your GP. You may want to take along a copy of your personal assessment when you see them.