The NHS offers screening to save lives from breast cancer. Screening does this by finding breast cancers at an early stage, when they're too small to see or feel.
But it does have some risks.
Screening also does not prevent you 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.
Breast screening helps identify breast cancer early. The earlier the condition is found, the better the chances of surviving it.
Some women who have screening will be diagnosed and treated for breast cancer that would never have otherwise caused them harm.
Read about breast cancer treatment, including potential side effects.
Following screening, about 1 in 25 women will be called back for further assessment.
Being called back does not 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 1 in 4 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 1 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.
But the benefits of screening and early detection are thought to outweigh the risks of having the X-ray.
There's debate about how many lives are saved by breast screening and how many women are diagnosed with cancers that would not have become life threatening.
The numbers below are the best estimates from a group of experts who have reviewed the evidence.
Screening saves about 1 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.
About 3 in every 200 women screened every 3 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.
Overall, for every 1 woman who has her life saved from breast cancer, about 3 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.
The causes of breast cancer are not fully understood, making it difficult to say why one woman may develop breast cancer and another may not.
But there are risk factors known to affect your likelihood of developing breast cancer.
Some of these you cannot do anything about, but there are some you can change.
Learn about the risk factors for breast cancer.