What is a mammogram? Screening, preparation and results

What a mammogram is

The US National Cancer Institute (NCI) describes mammography as an imaging test that uses low-dose x-rays to create pictures of the breast. Mammograms are used for breast cancer screening because they can find tumours at an earlier stage, before they cause symptoms (NCI: Mammograms). The NHS in England describes its breast screening in a similar way: x-rays called mammograms are used to look for cancers that are too small to see or feel (NHS: Breast screening (mammogram)).

There are two kinds of mammogram, made on the same machines:

• Screening mammogram - for people who have no symptoms, to look for early signs of cancer.
• Diagnostic mammogram - used after a person or their doctor finds a lump or another change. NCI explains that a diagnostic mammogram takes images from more angles, so the radiation dose is higher.

Screening and diagnosis are different steps. Our guide to cancer screening explained covers the general idea, and how cancer is diagnosed explains the tests that follow an abnormal result.

What a mammogram can show

A radiologist (a doctor who reads medical images) looks at the pictures for unusual changes and, where possible, compares them with earlier mammograms. NCI lists the main things a mammogram can show:

• A mass (lump). Its size, shape and edges give the radiologist information. NCI notes that a lump with a smooth, round shape and clear edges is often a benign (non-cancerous) cyst, while an irregular shape or jagged outline may need more tests.
• Calcifications - tiny deposits of calcium, unrelated to the calcium in your diet. Larger ones (macrocalcifications) are usually benign. Very small ones (microcalcifications) in certain patterns may be a sign of early changes or cancer, so they may be checked further.
• Breast density - how much dense tissue there is compared with fatty tissue. NCI notes that mammography is more likely to miss cancer in dense breasts.

Who is offered screening mammograms

Recommendations differ between countries and organisations, so the right answer for you depends on where you live and on your personal risk. Some examples from official sources checked today:

• United States: the US Preventive Services Task Force (USPSTF) recommends screening mammography every two years for women aged 40 to 74. It says the evidence is insufficient to judge the balance of benefits and harms for women aged 75 or older, and also for extra ultrasound or MRI in women whose only finding is dense breasts (USPSTF: Breast Cancer Screening, 2024).
• England: NHS breast screening is offered to women aged 50 up to their 71st birthday. The first invitation usually comes between 50 and 53, then every 3 years. People aged 71 or over are not invited automatically but can ask their local service for an appointment every 3 years (NHS: Who breast screening is for).
• Globally: the World Health Organization (WHO) describes population screening as mammography in apparently healthy women, usually aged 50 to 69 (WHO: Breast cancer fact sheet).

If your risk may be higher. NCI notes that some people at very high risk may be advised to start earlier and be screened more often - for example, people who had radiation to the chest for a childhood cancer, or people with harmful changes in the BRCA1 or BRCA2 genes. The NHS similarly says that people with a strong family history, certain gene changes or dense breast tissue may be invited before 50, invited more often, or offered MRI scans as well. If this may apply to you, speak to your doctor, and see our guide is cancer hereditary?

The NHS also points out that anyone can get breast cancer, including men, trans and non-binary people. Screening invitations depend on how you are registered, so if you are unsure whether you should be screened, ask your GP or doctor.

How to prepare for a mammogram

The NHS says you do not need to do anything special to prepare, but a few things can help (NHS: What happens at your appointment):

• Wear a top that is easy to remove, rather than a dress, because you will undress above the waist.
• Do not use deodorant, antiperspirant or talcum powder on the day, as these may affect the pictures. NCI gives the same advice and adds lotions, creams, powders and perfumes on the breasts or under the arms.
• Remove necklaces and nipple piercings.
• Tell the staff if you have difficulty standing or walking, if you found screening uncomfortable before, or if you feel nervous - they are trained to help.
• If possible, NCI suggests not booking the appointment just before or during your period, when breasts may be more tender.
• If you have breast implants, tell the facility in advance. NCI explains that implants can hide some breast tissue and that a special technique may be used.

What happens during the test

The NHS describes the appointment step by step. A specialist called a mammographer checks your details and asks about any breast problems. You undress above the waist in private. Each breast in turn is placed on the x-ray machine and gently pressed between two plastic plates for a few seconds while an x-ray is taken, then the machine is moved for a second picture. The x-rays take only a few minutes, and the whole appointment should take no longer than 30 minutes.

NCI explains that pressing the breast flat helps get a clearer picture. Both organisations say the pressure can be uncomfortable and sometimes painful for some people. The NHS adds that any discomfort should not last long, and that you can talk to the mammographer or ask to stop at any time.

3D mammograms (tomosynthesis)

You may see "3D mammogram" or "mammogram with tomo" on a booking. NCI explains that digital breast tomosynthesis (DBT) takes many pictures of thin slices of the breast and builds them into a 3D image. Combined with standard mammography, it is better at finding tumours than standard mammography alone, but NCI says it is still unknown whether it is better at reducing deaths from breast cancer, and a large trial is studying this question.

Getting your results

Both NCI and the NHS say results usually arrive within about two weeks. In the US, the report usually includes a BI-RADS category, a standard scale radiologists use. In simple terms, NCI describes category 0 as needing more images, 1 and 2 as negative or benign findings with routine screening continuing, 3 as probably benign with a short-term follow-up mammogram, 4 and 5 as findings that may need or require a biopsy, and 6 as a cancer already confirmed by biopsy. The report also includes information about breast density.

Being called back for more tests is common and is not a diagnosis. NCI notes that most people who are called back are not found to have breast cancer, and the NHS says most people who need further tests will not be diagnosed with breast cancer. Further tests may include a breast examination, more mammograms, an ultrasound scan, or a small sample taken with a needle (a biopsy) (NHS: Your breast screening results). Writing down questions to ask your doctor can help at a results appointment.

Benefits and limits

The NHS explains that finding breast cancer early can make successful treatment more likely and make it less likely that a breast needs to be removed. It also lists the limits honestly (NHS: Why breast screening is done):

• Overdiagnosis - some cancers found and treated would never have become life-threatening. The NHS calls this the main risk of breast screening.
• Missed cancers - mammograms do not always find a cancer that is there, and cancer can develop between mammograms.
• Radiation - the NHS says regular mammograms over many years give a very slightly higher lifetime chance of cancer because of the small amount of radiation.

Because of this, it is your choice whether to be screened, and it is reasonable to discuss the benefits and harms with your doctor first.

Sources

• National Cancer Institute: Mammograms
• US Preventive Services Task Force: Breast Cancer Screening (final recommendation, 2024)
• NHS: Who breast screening is for
• NHS: Why breast screening is done
• NHS: What happens at your breast screening appointment
• NHS: Your breast screening results
• World Health Organization: Breast cancer fact sheet

Published and last reviewed: 21 October 2026.