Risks for breast cancer

Some things can affect your risk, or chance, of developing cancer. Certain behaviours, substances or conditions can increase or decrease the risk. Most cancers are a result of many risks. But sometimes cancer develops in people who don't have any risks.

Anyone can develop breast cancer, because everyone has breast tissue. Men just have a lot less of it. Breast tissue in both women and men is exposed to the hormones estrogen and progesterone. The exposure in women is higher because they have higher levels of these hormones. Both hormones – especially estrogen – are linked with breast cancer and encourage the growth of some breast cancers.

The information on this page is mainly about risks in women. Most breast cancers occur in women, and most of the research on breast cancer has been in women. If we know from research that men also have the same risk, we say so.

There is not a lot of research on breast cancer risk in people who are transgender, non-binary or gender-diverse. We welcome research in this area. If you are transgender or non-binary and you're worried about your risk of breast cancer, talk to your healthcare team about how the information here applies to you and find out about screening in 2SLGBTQI+ communities.

Breast cancer is more common in high-income, developed countries such as Canada, the United States and some European countries. The risk of developing breast cancer increases with age. Breast cancer cases are highest in women between 50 and 69 years of age.

Most of the risks for breast cancer can't be changed. But in some cases, there are things that you can do to lower your risk.

If you have had breast cancer, you have a higher risk of developing a second breast cancer.

A family history of breast cancer means that one or more of your close blood relatives have or had breast cancer. Some families have more cases of breast cancer than would be expected by chance. Sometimes it isn't clear whether the family’s pattern of cancer is due to chance, shared lifestyle factors, genes passed from parents to children or a combination of these factors.

Your risk of developing breast cancer is higher if:

• one or more first-degree relatives (such as a mother, sister or daughter) had breast cancer, especially if they were diagnosed before menopause
• second-degree relatives (such as a grandmother, aunt or niece) on either side of your family had breast cancer
• a relative had cancer in both breasts (called bilateral breast cancer) before menopause
• 2 or more relatives had breast cancer or ovarian cancer
• a relative had both breast cancer and ovarian cancer
• a male relative had breast cancer

Having one first-degree relative with breast cancer approximately doubles your risk. The more first-degree relatives with breast cancer, the greater the risk. The risk with second-degree relatives is not as much as the risk with first-degree relatives.

Find out more about genetic risk and cancer.

Dense breasts have more connective tissue, glands and milk ducts than fatty tissue. Breast density is often inherited, but other factors can affect your breast density. You have a higher risk of developing breast cancer if you have dense breast tissue.

Find out more about breast density.

Genetic mutations are changes to a gene. Some gene changes can increase the risk of developing certain types of cancer. Inherited gene mutations are passed on from a parent to a child. Only a small number of breast cancers (about 5% to 10%) are caused by an inherited gene mutation.

BRCA1 (breast cancer gene 1) and BRCA2 (breast cancer gene 2) are genes that we all have in our DNA. They are called tumour suppressor genes because they appear to play a role in controlling the growth of cancer cells. Mutations in the BRCA1 or BRCA2 genes can affect the genes so they no longer control cancer growth. These mutations are rare. They occur in about 1 in 500 people.

You can inherit a mutated BRCA gene from either parent. People who have the gene mutation can also pass it on to their children. If one parent has the mutation in 1 of the 2 copies of the BRCA gene, a child has a 50% chance of inheriting the gene mutation. This also means there is a 50% chance that a child will not inherit the gene mutation.

The likelihood that breast or ovarian cancer is linked with an inherited BRCA1 or BRCA2 mutation is highest in families that have:

• a history of family members with breast or ovarian cancer
• breast cancer that developed before age 50 in 1 or more female relatives
• a relative diagnosed with both breast and ovarian cancer
• family members who develop cancer in both breasts
• a male relative diagnosed with breast cancer

Women with inherited BRCA1 or BRCA2 gene mutations have up to an 85% chance of developing breast cancer in their lifetime. They also have a higher risk of developing breast cancer at a younger age (usually before menopause) than other women. Women with a BRCA gene mutation also have a higher risk of developing cancer in both breasts. If they develop cancer in one breast, they have a greater risk of developing cancer in the other breast.

Men who have an inherited BRCA gene mutation are also at a higher risk of developing breast cancer. The risk is higher with BRCA2 mutations than BRCA1 mutations.

If several of your family members develop breast cancer, talk to your doctor about hereditary breast cancer and genetic testing.

A genetic condition is a disease caused by a change (mutation) in one or more genes. Having any of the following genetic conditions increases your risk of developing breast cancer.

Li-Fraumeni syndrome is an inherited condition that is associated with an increased risk of developing certain types of cancers, including breast cancer, brain tumours, acute leukemia, soft tissue and bone sarcomas, and adrenal cortical carcinomas. People with Li-Fraumeni syndrome tend to develop several different types of cancer before the age of 45.

Ataxia-telangiectasia is an inherited disease that affects the nervous system, immune system and other body systems. Signs and symptoms include loss of balance, poor coordination, frequent infections, red eyes (due to widening of blood vessels) and abnormal eye movements. Ataxia-telangiectasia is associated with an increased risk of developing some cancers, including breast cancer, leukemia and lymphoma.

Cowden syndrome is an inherited condition in which many non-cancerous (benign) growths (hamartomas) form in the skin, breast, thyroid, colon and intestines and inside the mouth. People with this condition are more likely to develop breast cancer, gastrointestinal cancers and thyroid cancer.

Peutz-Jeghers syndrome is an inherited condition that causes dark spots on the mouth and fingers, and polyps in the large and small intestines. Peutz-Jeghers syndrome increases the risk of developing breast cancer, as well as colorectal, small intestine, stomach, pancreatic and other types of cancer.

Klinefelter syndrome is a genetic condition in which a man has an extra X sex chromosome. Men normally have an X and a Y chromosome. Someone with Klinefelter syndrome has 2 X chromosomes and a Y chromosome. People with this condition have an increased risk of breast and testicular cancer.

The following gene mutations are linked with a higher risk for breast cancer.

CHEK2 gene mutation has been identified in some families with Li-Fraumeni syndrome. CHEK2 is a tumour suppressor gene. When it is mutated, it increases breast cancer risk.

PALB2 gene mutations cause the PALB2 gene to stop working. The PALB2 gene normally protects against breast cancer by helping repair changes to DNA and preventing tumour growth. When this gene loses its function, there is a higher risk for breast cancer.

Women of Ashkenazi Jewish descent have a higher risk of developing breast cancer. This is because BRCA1 and BRCA2 gene mutations are more common in Ashkenazi women. About 1 in 40 Ashkenazi Jewish women carry a BRCA gene mutation, while only 1 in 500 women in the general population have it.

Estrogen plays a crucial role in the development and function of the female reproductive system, including the menstrual cycle and pregnancy. Experts believe that estrogen also plays an important role in the growth of breast cancer cells. The type of exposure and how long cells are exposed to estrogen affects the chances that breast cancer will develop.

Getting your monthly period for the first time at an early age (11 or younger) means that your cells are exposed to estrogen and other hormones for a greater amount of time over your lifetime. This leads to a small increase in your risk of breast cancer. Doctors call the start of menstruation "menarche" and getting your period at a young age "early menarche."

Menopause is when your monthly periods stop. It occurs as the ovaries stop making hormones and the level of hormones (mainly estrogen and progesterone) in the body drops. Once you have gone through 12 months without getting your period, you are post-menopausal. For most women, their periods stop somewhere between the ages of 45 and 55.

If your periods stop at a later age (after age 55), breast cells are exposed to estrogen and other hormones for a longer amount of time. This increases your risk of breast cancer. Early menopause (before age 45) is linked with a lower risk of breast cancer.

Having your first full-term pregnancy after the age of 35 means that you have a higher risk of breast cancer than someone who has at least one full-term pregnancy at an earlier age. Becoming pregnant at an early age (such as before age 20) reduces breast cancer risk.

There is an increased risk of breast cancer for about 10 years after having a first child. But after 10 years, women who gave birth have a lower risk of breast cancer than women who never gave birth.

Not becoming pregnant and giving birth at all (called nulliparity) increases your risk for breast cancer.

A woman has a lower risk of breast cancer when she has given birth. The more times a woman gives birth the lower her risk of breast cancer. After each child, the risk of breast cancer is lowered. Women who have childbirths spaced closely together may also have a lower risk of breast cancer than those women who have childbirths spaced further apart.

Breastfeeding can lower the total number of menstrual cycles your body has over your lifetime and reduces the amount of estrogen that breast cells are exposed to. Women who don't breastfeed have a higher risk of breast cancer than women who breastfeed.

There are many different reasons why you may choose not to breastfeed, and it may not be possible for you to breastfeed your baby.

Being exposed to ionizing radiation increases the risk of developing breast cancer.

Women who were given a particular type of radiation therapy called Mantle field radiation to the chest, neck and armpit area for Hodgkin lymphoma are at a higher risk of developing breast cancer. This risk is high if the treatment was given before the age of 30, and it is highest when treatment was given during puberty. Breast cancer risk is greatly increased if chemotherapy and radiation therapy were used to treat Hodgkin lymphoma before the age of 15 years. But the benefit of treating the cancer usually far outweighs the risk of developing a second cancer from radiation therapy treatment. Today, radiation therapy is given to the smallest possible area of the body and this reduces the risk for long-term side effects, including breast cancer risk.

In the past, radiation therapy was used to treat health problems such as tuberculosis, acne or an enlarged thymus gland. Women who were given radiation therapy to the chest area for these diseases have a greater risk of developing breast cancer.

Women exposed to ionizing radiation during atomic bomb blasts during the Second World War also have a higher risk of developing breast cancer, especially if they were exposed during puberty.

Hormone replacement therapy (HRT) uses the hormones estrogen or progesterone or both to manage the symptoms of menopause, such as hot flashes, vaginal dryness and mood swings.

Taking HRT for a long time increases the risk of breast cancer. This is especially true for HRT that uses both hormones (called combined HRT). Studies have shown that current or recent users of combined HRT for 5 years or longer have a higher risk for breast cancer.

Learn more about the benefits and risk of hormone replacement therapy.

Birth control pills (oral contraceptives) contain the hormones estrogen and progesterone. The hormones work together to prevent pregnancy by blocking the release of eggs from the ovaries. People take them for other health-related reasons as well.

Birth control pills increase your risk for breast cancer, especially if you have used them for 10 or more years. This higher risk disappears after you stop taking birth control pills. However, if you currently use them or have recently used them (less than 10 years since last use) you have a slightly greater risk compared with someone who has never used birth control pills.

Learn more about the benefits and risks of hormonal birth control.

Atypical hyperplasia is a non-cancerous (benign) condition where there is a greater number of abnormal (atypical) cells in the breast tissue. Atypical hyperplasia increases your risk of developing breast cancer.

Drinking alcohol increases your risk for breast cancer. The more you drink, the greater your risk.

Learn more about how to limit alcohol.

Overweight and obesity increase the risk for breast cancer in post-menopausal women. Studies show that the risk of breast cancer in post-menopausal women increases as weight gain increases.

The ovaries make most of the body’s estrogen, but after menopause fat tissue produces a small amount of estrogen. Having more fat tissue can increase estrogen levels and so increase the chance that breast cancer will develop.

Learn more about how to have a healthy body weight.

Sendentary behaviour, or sitting for long periods of time, increases your risk for breast cancer.

Learn more about how to move more, sit less.

Breast cancer risk is slightly higher for women with higher incomes. This may be because of other factors that are linked to breast cancer, such as having children later in life or having fewer children.

Tall women have a slightly higher risk of developing breast cancer after menopause. The amount and type of food and drink you consume early in life affect adult height. It is thought that these are the factors that increase the risk, rather than being tall.

Tall women may also have a higher risk of developing breast cancer before menopause, but more research is needed to confirm this.

Women who gain weight as an adult are at an increased risk of developing breast cancer after menopause.

Find out more on how to have a healthy body weight.