Risk factors for breast cancer

Photo of a mother and daughter  (PantherMedia / Dubova)

Most women have some risk of getting breast cancer. But many risk factors, such as lifestyle, play only a small role. The biggest risk factors are your genes (family history) and age.

In recent decades there has been a lot of intensive research on the causes of breast cancer and other cancers. It is now clear that there is no one cause of breast cancer. Whether or not a woman develops breast cancer will mainly depend on the following factors:

  1. Age and medical history. The likelihood of developing cancer increases as you grow older. This is because gene mutations (changes) start becoming more likely, while at the same time certain protective mechanisms stop working properly. Women who have already had breast cancer are more likely to get it in the future.
  2. Genetic predisposition. Cancer arises when genes in a cell change (mutate). The body can usually prevent or repair such changes. In people who are more likely to get cancer because of their genes, these protective mechanisms aren't as effective.
  3. Hormones. Hormones can influence how cancer cells grow. Hormone-related factors that play a role in breast cancer include the age at which a woman had her first period, when she had her first child, and how often she was pregnant.
  4. Lifestyle. Certain lifestyle factors and life circumstances can affect your risk of breast cancer, but they tend to play a minor roll.

There are so many different risk factors that most women have several of them at the same time. This is normally no real cause for concern, though, because most of these factors only slightly increase the risk and are quite "normal." So having several risk factors doesn't necessarily mean you're more likely to get breast cancer. Many women who develop breast cancer have the same risk factors as women who don't get this disease.

Whether or not a woman develops breast cancer is also a matter of luck: Sometimes gene mutations occur spontaneously, for no particular reason.

The influence of age

The following table shows how the risk of getting breast cancer generally changes as you grow older.

Age Number of women who will develop breast cancer
within the next ten years
30 years old 5 out of 1,000
40 years old 15 out of 1,000
50 years old 25 out of 1,000
60 years old 36 out of 1,000
70 years old 34 out of 1,000

Overall, about 130 out of 1,000 women will get breast cancer in their lifetime. And 1 out of 1,000 men will get this disease at some point in their lives too.

The influence of genes

Many women with one or more relatives who have had breast cancer wonder how that might affect their own risk. And what role genes might play. The answers will greatly depend on details:

  • For instance, if your grandmother or cousin develops breast cancer (i.e. a second-degree relative or a third-degree relative), your own risk is hardly affected.
  • If your mother or sister (a first-degree relative) develops the disease, that doubles your risk. But if only one of your close relatives has had breast cancer, that doesn't necessarily mean that you have a genetic predisposition caused by a mutated gene. Gene tests usually don't provide any useful information.
  • If, for example, several close relatives have breast cancer, that may indeed suggest that their cancer was caused by a genetic mutation passed on in the family. When is genetic testing a good idea? About 5 to 10% of all breast cancer cases are known to be caused by gene mutations (changes). They are known as "hereditary breast cancer." Particularly mutations affecting the genes known as BRCA1 and BRCA2 increase the risk of breast cancer quite a lot: About 65% of women with BRCA1 mutations develop breast cancer before the age of 69, compared to about 45% of women with BRCA2 mutations. Counseling and support is currently offered by 17 "hereditary breast and ovarian cancer (HBOC) centers" in Germany.

What role do hormones play?

Hormonal changes throughout the menstrual cycle also influence breast tissue. In particular, the gland tissue responsible for making milk grows every month and then breaks down again if the woman doesn't get pregnant.

The growth and behavior of this tissue are influenced by sex hormones such as estrogens and progestin. So factors that are related to hormone levels can also influence your risk of breast cancer. These include the following:

  • Getting your first period at a young age
  • Late menopause
  • Taking hormones
  • Taking the contraceptive pill
  • Never having given birth
  • Being older than 30 when you give birth to your first child

A high breast density (i.e. the breast mainly consists of glandular tissue) can also influence the risk of breast cancer. Although breast density isn't determined by hormones, it can be influenced by them.

What role do your lifestyle and environment play?

Research has shown that lifestyle factors hardly affect your risk of breast cancer. Some studies have found that the risk is somewhat smaller in women who are very physically active. There is also some emerging evidence which suggests that a certain kind of diet – a Mediterranean diet – might be associated with a somewhat lower risk. But otherwise research has so far supported the belief that your risk of breast cancer isn’t affected by what you eat or drink.

One exception is alcohol: Women who drink a lot of alcohol are more likely to get breast cancer. It isn’t yet clear whether smoking has the same effect.

The influence of body weight hasn’t been properly determined either. Studies have shown that breast cancer is somewhat more common in women who become overweight after menopause (BMI over 25). The opposite is true before menopause, when being overweight is actually associated with a slightly lower risk of breast cancer. It is not known why this is.

Some factors are claimed to increase the risk. These include using deodorants with aluminium in them, wearing (tight) bras, having breast implants, stress, and having had abortions in the past. But there is no scientific evidence to support these claims.

What does “increased risk” actually mean?

Things like “increased risk” or “more likely to get breast cancer” may sound scary if they aren’t explained in more detail. To be able to assess how high your own risk actually is, you have to know what baseline risk the “increased risk” is based on.

The following table shows how many fifty-year-old women with certain risk factors are expected to get breast cancer within the next ten years:

Risk factors Number of fifty-year-old women who will develop breast cancer within the next ten years
  • no other significant risk factors and
  • very low breast density
10 out of 1,000
  • Average risk for fifty-year-old women
25 out of 1,000
  • high breast density (category 3) and
  • mother had breast cancer
43 out of 1,000
  • breast changes that have already been checked in the past (biopsy) and
  • very high breast density (category 4) and
  • mother had breast cancer
72 out of 1,000

How is your personal risk estimated?

Breast cancer risk calculators can be used to roughly determine your own personal breast cancer risk. These are based on studies involving women in the United States. But they can also give women in Germany and other countries a general idea of their risk. There are a number of different calculators that use different kinds of information to calculate the risk. 

The Breast Cancer Surveillance Consortium’s calculator

Information used:

  • Whether you have already had breast cancer in the past
  • Age
  • Race/ethnicity (for instance, the color of your skin)
  • Other cases of breast cancer in your family
  • Breast changes that have been checked in the past (biopsy)
  • Breast density
The National Cancer Institute’s calculator

Information used:

  • Whether you have already had breast cancer in the past
  • Hereditary breast cancer
  • Age
  • Age when you got your first period
  • Age when you gave birth to your first child
  • Other cases of breast cancer in your family
  • Breast changes that have been checked in the past (biopsy)
  • Race/ethnicity