Mammogram Basics

A mammogram is a low-dose x-ray that allows doctors called radiologists to look for changes in breast tissue.

Why do I need mammograms?

A mammogram can often find or detect breast cancer early, when it’s small and even before a lump can be felt. This is when it’s easiest to treat.

What are the types of mammograms?

Screening mammograms

A screening mammogram is used to look for signs of breast cancer in women who don’t have any breast symptoms or problems. X-ray pictures of each breast are taken, typically from 2 different angles.

Diagnostic mammograms

Mammograms can also be used to look at a woman’s breast if she has breast symptoms or if a change is seen on a screening mammogram. When used in this way, they are called diagnostic mammograms. They may include extra views (images) of the breast that aren’t part of screening mammograms. Sometimes diagnostic mammograms are used to screen women who were treated for breast cancer in the past.

What do mammograms show?

Mammograms can often show abnormal areas in the breast. They can’t prove that an abnormal area is cancer, but they can help health care providers decide whether more testing is needed. The 2 main types of breast changes found with a mammogram are calcifications and masses. Learn more about these and other breast changes in What Does the Doctor Look for on a Mammogram?

How do mammograms work?

A mammogram uses a machine designed to look only at breast tissue. The machine takes x-rays at lower doses than usual x-rays. Because these x-rays don’t go through tissue easily, the machine has 2 plates that compress or flatten the breast to spread the tissue apart. This gives a better picture and allows less radiation to be used.

To learn more about how they are done, see Mammograms: What to Know Before You Go.

illustration showing a woman getting a mammogram

In the past, mammograms were typically printed on large sheets of film. Today, digital mammograms (also known as full-field digital mammography or FFDM) are much more common. Digital images are recorded and saved as files in a computer. 

Breast tomosynthesis (also known as digital breast tomosynthesis [DBT] or 3D mammography): For this newer type of mammogram, the breast is compressed once, and a machine takes many low-dose x-rays as it moves in an arc over the breast. A computer then puts the images together into a series of thin slices. This allows doctors to see the breast tissues more clearly. Some studies have suggested it can lower the chance of being called back for follow-up testing. It may also be able to find more cancers, especially in women with more dense breasts. But this test is not yet available at all imaging centers.

Are mammograms safe?

Mammograms expose the breasts to small amounts of radiation. But the benefits of mammography outweigh any possible harm from the radiation exposure. Modern machines use low radiation doses to get breast x-rays that are high in image quality. On average the total dose for a typical mammogram with 2 views of each breast is about 0.4 millisieverts, or mSv. (A mSv is a measure of radiation dose.) The radiation dose from breast tomosynthesis can range from slightly lower to slightly higher than that from standard mammograms.

To put these doses into perspective, people in the US are normally exposed to an average of about 3 mSv of radiation each year just from their natural surroundings. (This is called background radiation.) The dose of radiation used for a screening mammogram of both breasts is about the same amount of radiation a woman would get from her natural surroundings over about 7 weeks.

If there’s any chance you might be pregnant, let your health care provider and x-ray technologist know. Although the risk to the fetus is very small, and mammograms are generally thought to be safe during pregnancy, screening mammograms aren’t routinely done in pregnant women who aren't at increased risk for breast cancer.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

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Last Medical Review: October 3, 2019 Last Revised: October 3, 2019

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