Breast Asymmetry

What is breast asymmetry?

Breast asymmetry means your breasts aren’t exactly symmetrical — that is, exactly the same on each side. This is common and usually nothing to worry about. But your healthcare provider might make a note of it after examining your breasts. They do this to keep track of what’s normal for you and what might be a change. If breast asymmetry is a recent development, there could be a medical reason for it.

Your provider might notice:

• Differences in breast size: For example, one breast might be a size larger than the other. Your primary care provider might notice it in a clinical exam. Or it might show up on an imaging test.
• Differences in breast shape: If one of your breasts has an abnormal shape, like a bulge or swelling on one side, your provider might want to investigate. Sometimes, it’s abnormal tissue.

Asymmetry on a mammogram

When you have a mammogram, the radiologist who reads the images might notice a difference in tissue density between your two breasts. Dense breast tissue is one of the things mammograms check for.

Your mammogram report might note:

• Asymmetry: This note usually means they found an isolated area of dense breast tissue in only one view on your mammogram. You might need another test to confirm it’s really there.
• Focal asymmetry: This is an isolated area of dense breast tissue that they saw in two different views on your mammogram. “Focal” means it was in one spot and they can verify the location.
• Global asymmetry: This is a large area of dense breast tissue that’s visible in two different views on your mammogram. It covers a large part of your breast — about one-fourth or more.
• Developing asymmetry: This means they noticed a difference in breast density that wasn’t there on previous mammograms. As it’s a change, your provider will want to evaluate it further.

“Asymmetry” on your mammogram report only refers to differences in breast density. If they notice other differences in shape or size, they’ll call them something else. But these can also be important.

What causes breast asymmetry?

To start with, almost no one has perfectly symmetrical breasts. But if you have more noticeable breast asymmetry, or it appears on your mammogram report, there could be several reasons for it.

Often, breast asymmetry develops when your breasts develop in puberty. This means it’s written in your genes. Temporary hormonal changes, like during your menstrual cycle, pregnancy or hormone replacement therapy, can also make your breasts grow — sometimes, unevenly. But if your breasts haven’t grown recently, and you notice a new asymmetry, there might be a medical cause.

Possible medical causes include:

• Breast cysts: A cyst can develop when fluid fills an empty milk duct in your breast. Some cysts can grow large, and some can grow in clusters in one breast, affecting the size and shape.
• Breast hematoma: A hematoma is a collection of blood inside your tissue, like a swollen bruise. It can happen after an injury. It’s temporary and goes away by itself, without treatment.
• Fat necrosis: Necrosis occurs when fat tissue in your breast dies, usually from an injury. The tissue goes through several stages of changes, which can include a lump or thickening.
• Tumors: Rarely, a tumor or abnormal growth may show up as a change in the size, shape or contour of your breast. Tumors can be malignant (cancer) or benign (not cancer).

How common is breast asymmetry?

Having breasts that look slightly asymmetrical by nature is very common. It varies a little with ethnicity. For example, it’s slightly more common in Black people and less common in Chinese people.

Asymmetry in breast density appears as an initial finding in about 7% of mammogram reports. But as many as half of these findings may be an illusion. It happens when breast tissue overlaps in the image.

How is this symptom treated?

If your provider finds a new asymmetry on your mammogram or another exam, they’ll follow up with other tests to find out what’s causing it. It’s usually benign, but they’ll want to rule out breast cancer.

Additional tests might include:

• 3D mammography (tomosynthesis)
• Breast ultrasound
• Breast MRI
• Molecular breast imaging
• Breast biopsy

Once they’ve found the cause, they’ll treat it as needed. In many cases, you won’t need treatment.

If you don't have a condition that needs treatment, but you want to address breast asymmetry for cosmetic reasons, you can talk to a cosmetic surgeon. They can offer surgical and nonsurgical options.

How often is breast asymmetry cancer?

The chance of breast asymmetry discovered on a screening mammogram turning out to be cancer is about 12%. The chance goes up to about 27% if your follow-up mammogram confirms the asymmetry.

When should I see my healthcare provider about asymmetrical breasts?

If you notice a change in your breasts that makes one look or feel different from the other, it’s a good idea to see a healthcare provider. They’ll likely recommend a mammogram to check it out. Breast changes can occur for harmless reasons, but it’s good to know what the reason is and have it in your medical records. That way, your healthcare provider can keep track of what’s normal for your breasts.