A mammogram is a low-dose X-ray of the breast, used both to screen for breast cancer in women who have no symptoms and to investigate a lump, pain, or other breast change.
Mammograms are the most widely used imaging test for breast cancer. There are two broad purposes: screening mammograms are offered at regular intervals to people without symptoms to pick up cancers early, and diagnostic mammograms are done when there is a specific concern, such as a lump, and are usually combined with ultrasound and sometimes a biopsy in the same visit.
Orientation Why you might be reading about this
You have been invited for a screening mammogram, referred for a diagnostic mammogram because of a breast change, or are weighing up whether to arrange a private mammogram. This page explains what a mammogram is, what to expect on the day, and what the results can and cannot tell you.
Related terms: Breast ultrasound · Breast MRI · Tomosynthesis · Triple assessment · Microcalcification · Breast density
The procedure How a mammogram is done
A mammogram is taken by a specially trained radiographer using a dedicated mammography machine. The breast is placed onto a flat plate and a second plate is lowered to compress it gently. Two views are taken of each breast — one from above and one from the side. Compression spreads the breast tissue so that small abnormalities are easier to see and reduces the X-ray dose needed. Each compression lasts only a few seconds; most people describe it as uncomfortable but brief. The whole appointment usually takes less than 20 minutes.
Two purposes Screening versus diagnostic mammograms
A screening mammogram is offered at regular intervals to people without breast symptoms, to pick up early breast cancer before it is felt as a lump. In the UK, the NHS Breast Screening Programme invites women aged 50 to 71 every three years, with the first invitation between ages 50 and 531, with older women able to self-refer. The programme is supported by independent UK review evidence of meaningful breast cancer mortality reduction3.
A diagnostic mammogram is performed when there is a specific concern — a lump, pain, a change in the breast or nipple, or follow-up after a previous finding. Diagnostic mammograms are usually combined with ultrasound and, where indicated, a biopsy as part of a triple assessment in a one-stop breast clinic. For symptomatic women, mammography is the first-line investigation from age 40 (ultrasound is preferred under 40), per Royal College of Radiologists guidance.
3D mammography Tomosynthesis (3D mammography)
Most modern mammography machines can also take tomosynthesis images — a series of thin-slice views that the radiologist reads as a 3D reconstruction of the breast. Tomosynthesis improves the detection of some cancers, particularly in women with dense breast tissue, and reduces the rate of unnecessary recalls4.
Limitations What a mammogram can and cannot show
A mammogram is very good at picking up microcalcifications (the tiny specks of calcium often associated with DCIS) and at showing masses with typical cancerous features. It is less reliable in women with dense breast tissue, where cancers can be harder to distinguish from the surrounding tissue2. For this reason, mammography is often combined with ultrasound — and in selected patients with MRI — rather than relied on alone5.
A normal mammogram is reassuring but not an absolute guarantee. Any breast change that persists — a lump, skin change, nipple change — should still be examined, even if a recent mammogram was normal.
What to expect What to expect at your appointment
- Bring any previous mammograms or imaging if you have them — comparison with prior imaging significantly improves interpretation.
- You will be asked to undress to the waist, so a two-piece outfit is easier.
- Deodorant, body lotion, or talc on the chest area can show up on the X-ray as small white dots and lead to a recall, so avoid them on the day.
- Results from a screening mammogram are usually posted within two weeks1. Results from a diagnostic mammogram done as part of a one-stop assessment are usually discussed on the same day.
Read more on the imaging services page and the one-stop clinic page.
Resources Further reading
- NHS Breast Screening (mammogram) — UK NHS overview of who is invited, when, and what happens at a screening appointment.
- NHS — Mammogram (breast X-ray) — patient guide to what a mammogram is and what the results mean.
- Public Health England / UK Health Security Agency — Breast screening leaflet — the official screening information leaflet.
- Breastory: Imaging services · One-stop breast clinic · Tomosynthesis