The NHS offers screening to save lives from breast cancer. Screening does this by finding breast cancers at an early stage, when they are too small to see or feel.
However, it does have some risks. Some women who have screening will be diagnosed and treated for breast cancer that would never otherwise have caused them harm.
Screening does not prevent you from getting breast cancer, and it may not help if you already have advanced stage breast cancer.
It's up to you to decide if you want to have breast screening.
The risks and benefits are explained in more detail below.
Benefits of breast screening
Most experts agree that regular breast screening is beneficial in identifying breast cancer early. The earlier the condition is found, the better the chances of surviving it.
You're also less likely to need a mastectomy (breast removal) or chemotherapy if breast cancer is detected at an early stage.
Risks of breast screening
Some women who have screening will be diagnosed and treated for breast cancer that would never otherwise have caused them harm. Read about breast cancer treatment, including potential side effects.
Following screening, about one in 25 women will be called back for further assessment. Being called back doesn't mean you definitely have cancer. The first mammogram may have been unclear.
Most women who receive an abnormal screening result are found not to have breast cancer. These women may experience unnecessary worry and distress.
About one in four women who are called back for further assessment are diagnosed with breast cancer.
There's a small chance that you'll receive a negative (all clear) mammogram result when cancer is present. Breast screening picks up most breast cancers, but it misses breast cancer in about one in 2,500 women screened.
A mammogram is a type of X-ray, and X-rays can, very rarely, cause cancer.
During a mammogram, your breasts are exposed to a small amount of radiation (0.4 millisieverts, or mSv).
For comparison, in the UK, a person receives a dose of 2.2 mSv a year from natural background radiation. However, the benefits of screening and early detection are thought to outweigh the risks of having the X-ray.
Weighing up the possible benefits and risks of breast screening
There is debate about how many lives are saved by breast screening and how many women are diagnosed with cancers that wouldn't have become life-threatening. The numbers below are the best estimates from a group of experts who have reviewed the evidence.
Saving lives from breast cancer
Screening saves about one life from breast cancer for every 200 women who are screened. This adds up to about 1,300 lives saved from breast cancer each year in the UK.
Finding cancers that would never have caused a woman harm
About three in every 200 women screened every three years from the age of 50 to 70 are diagnosed with a cancer that would never have been found without screening and would never have become life-threatening. This adds up to about 4,000 women each year in the UK who are offered treatment they did not need.
What this means
Overall, for every one woman who has her life saved from breast cancer, about three women are diagnosed with a cancer that would never have become life-threatening.
Researchers are trying to find better ways to tell which women have breast cancers that will be life-threatening and which women have cancers that will not.