What Is Breast Asymmetry?

Just as one of your feet may be slightly bigger than the other, it’s normal to have breasts that vary in size. The shape, position on your body, and density of one breast can also be slightly different from the other. These differences are called breast asymmetry.

If asymmetry between your breasts is bothering you, you may want to talk with your doctor. However, breast asymmetry is rarely a sign of a problem, according to Wendie A. Berg, MD, PhD. Berg, who specializes in breast imaging, is a distinguished professor of radiology at the University of Pittsburgh School of Medicine and Magee-Womens Hospital. Breast asymmetry, in and of itself, does not increase a person’s risk for breast cancer, Berg says.

Here’s some information to help you understand breast asymmetry.

Your breasts can look or feel different for a variety of reasons, including:

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  they’re still growing (for most people, breasts are fully developed by their early 20s) 

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  your hormones are changing due to ovulation, pregnancy, breastfeeding, taking hormonal contraceptives, or menopause

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  you’ve had surgery, trauma, burns, or infection that led to breast scarring or damage 

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  you’re missing a pectoral muscle on one side (called Poland syndrome) 

If there are areas of asymmetry in your breasts, a mammogram will reveal it.

Nearly half of women over 40 who receive mammograms are told they have dense breast tissue. While breast asymmetry does not increase the risk of breast cancer, greater breast density is a risk factor for breast cancer. People with dense breast tissues are 4 to 6 times as likely to develop breast cancer as people who don’t have dense breasts.

A routine mammogram may reveal one area of dense breast tissue that looks different from the same area on the other breast. While this is rare, it’s something that doctors who read mammograms (called radiologists) look for and may flag for follow-up. This information will appear in the mammogram report that goes to your doctor — called the Breast Imaging Reporting and Data System (BI-RADS).

The BI-RADS groups asymmetries into four distinct types to help radiologists make their assessments. If your mammogram identifies an area of asymmetry, you may see these terms on your mammogram report or hear your doctor use these terms to explain your report to you:

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  Asymmetry: An area of dense breast tissue visible in only one view of your mammogram. 

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  Focal asymmetry: An area of dense breast tissue visible in two views of your mammogram.

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  Global asymmetry: A large area of dense breast tissue visible in two views of your mammogram.

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  Developing asymmetry: When a new asymmetry is observed on your mammogram that wasn’t present in previous imaging, signaling a potential area of concern.  

“Most asymmetries are due to overlapping normal tissues,” says Berg, who is also the chief scientific advisor for densebreast-info.org. Sudden changes in the breast, though, are more concerning, she continues. For instance, if one breast shrinks and becomes harder, it may be a sign of diffuse invasive lobular cancer. And, if one breast rapidly becomes larger and develops redness and skin thickening (like an orange peel), it may be a sign of infection or inflammatory breast cancer.

Usually if a developing asymmetry is found, your doctor will review your past mammograms to better understand what might be happening. You’ll likely need to go in for another mammogram or more imaging such as an ultrasound or MRI. Breast MRI can also detect areas of asymmetry in the breasts.

If these tests suggest there’s a suspicious area, your doctor may order a biopsy. About 12% of developing asymmetry spotted on a screening mammogram turn out to be cancer.  

If follow-up breast imaging determines the breast asymmetry flagged on your mammogram isn’t cancer, your doctor will likely recommend that you continue routine breast screening. If an asymmetry is flagged on your first mammogram, the radiologist may recommend that you have a follow-up mammogram in six months. This will allow the doctor to confirm that the area of asymmetry is unchanged and represents normal tissue and not a developing asymmetry.

While focal asymmetry is rarely a sign of cancer, it requires careful assessment to rule out any underlying issues. If the focal asymmetry is stable over time, regular breast screenings will continue as usual, but a radiologist may pay extra attention to this area during future exams.

For people with developing asymmetry, where a noticeable change in breast size or shape has occurred over time, a doctor may recommend more frequent breast screenings to track any further changes. This approach will help your healthcare team to spot and address any issues as quickly as possible.