The right and left sides of the body are typically similar in shape and size, a quality known as symmetry. However, slight differences in the size and shape of the left and right breasts, known as breast asymmetry, are common and typically not a cause for concern.

However, if you have sudden changes in the size, shape, or texture of one breast, or if a mammogram detects a significant asymmetry, your healthcare provider may recommend further testing. In this article, we’ll explore the possible causes of breast asymmetry and when it may be important to seek medical advice.

What Causes Breast Asymmetry?

Breast asymmetry is very common. Up to 81 percent of women have noticeable differences between their breasts. Most of the time, breast asymmetry isn’t linked to cancer.

Breast size may differ while the breasts are still developing, which can continue into the early 20s. Other potential causes of breast asymmetry include:

• Weight changes (gaining or losing weight)
• Genetics
• Hormonal changes during the menstrual cycle, pregnancy, or menopause
• Surgery
• Injury or trauma to the breast
• Health conditions that affect the muscle or breast tissue

Breast asymmetry alone isn’t considered a risk factor for breast cancer. It may be perfectly normal for you to have asymmetric breasts.

It’s important to be familiar with how both breasts look and feel so you can recognize any changes. For many people, having asymmetrical breasts is completely normal.

Breast asymmetry alone isn’t considered a breast cancer risk factor. However, the only way to know that breast asymmetry isn’t caused by cancer is with additional testing.

Can Breast Cancer Cause Breast Asymmetry?

When breast cancer develops in one breast, the tumor may change the size and shape of that breast, making it look different from the other. These changes often appear suddenly and are different from your normal breast appearance.

Breast cancer symptoms that can cause differences in how one breast looks and feels include:

• Breast lump
• Swelling
• Hardness or firmness
• Shrinking of one breast
• Skin dimpling (indentations or puckering of the skin)
• Skin discoloration or rash
• Skin thickening
• Changes in nippled position or appearance

Breast changes aren’t always caused by breast cancer. Many other conditions can cause symptoms in one or both breasts. Regular screening for breast cancer can help find these changes in the early stages.

Breast Cancer Treatment and Breast Asymmetry

Breast cancer treatments can also cause breast asymmetry. Surgery is part of most breast cancer treatment plans and can result in changes in breast size and shape.

• A mastectomy involves removing the entire breast, leading to a noticeable difference in breast appearance.
• Lumpectomy removes only part of the breast but can still create size differences between the two breasts.

It’s important to be familiar with how both breasts look and feel so you know what’s normal for you. If you notice any sudden changes, talk to your doctor.

Radiation therapy for breast cancer can also affect breast shape and texture. Immediately after treatment, one breast may be larger due to swelling. Normally, the swelling gets better with time. Radiation therapy can also cause permanent changes to the affected breast by damaging healthy breast tissue. This damage can cause breast tissue to shrink or become thicker and harder. When this happens, the affected breast may not match the other breast as it naturally grows and shrinks with hormone or weight changes.

How Is Breast Asymmetry Detected?

If you have breast asymmetry along with symptoms that could indicate breast cancer, your doctor may recommend additional testing. However, breast asymmetry isn’t always noticeable just by looking. In many cases, it is first detected through breast cancer screening tests, such as a mammogram.

On a mammogram, breast asymmetry appears as an area of abnormal breast density that looks different from the surrounding tissue. To get a clearer view, radiologists typically take multiple images from different angles to examine the breast tissue from various perspectives.

Radiologists (specialists who analyze medical imaging) look for breast asymmetry when they review your mammogram. If asymmetry is found, the radiologist will:

• Describe the findings in a report sent to your healthcare provider
• Assign a score using the Breast Imaging Reporting and Data System (BI-RADS) scale, which helps guide next steps in diagnosis and management

About 3 percent of mammograms show breast asymmetry. Most of the time, breast asymmetry on a mammogram is caused by overlapping breast tissues rather than a serious condition. However, about 12 percent of breast asymmetries found with a mammogram turn out to be caused by breast cancer.

Types of Breast Asymmetries

Radiologists classify breast asymmetries based on different characteristics of the abnormality.

If your mammogram shows certain types of asymmetry, you may be recommended to undergo further testing.

There are four main types of breast asymmetries that may be included in the BI-RADS report.

Asymmetry

Asymmetry is defined as finding an area of dense breast tissue that can only be seen in one view of the mammogram.

Focal Asymmetry

A focal asymmetry is an area of dense breast tissue in one breast when compared to the corresponding area in the opposite breast, that can be seen in two different views of the mammogram. Focal asymmetries are usually benign (not cancerous). However, if new or changing, additional imaging may be recommended.

Global Asymmetry

Similar to focal asymmetry, global asymmetry is used to describe an area of dense breast tissue seen in two views of a mammogram. In global asymmetry, the area of dense breast tissue is large involving at least one-quarter of the breast.

Most of the time, global asymmetry is caused by previous treatments and benign conditions, such as:

• Overlapping tissue
• Surgery
• Cysts
• Hormone replacement therapy
• Oral contraceptives
• Diabetes

If global asymmetry is breast cancer, the most common types are invasive ductal carcinoma, invasive lobular carcinoma, and lymphoma.

Developing Asymmetry

A radiologist may describe an area of dense breast tissue as a developing asymmetry if it wasn’t there in a previous mammogram. This term can also be used to describe an area of dense tissue that has grown larger or developed additional concerning features (such as changes in shape, size, or texture). In most cases, a developing asymmetry requires more testing.

What Happens if You Have Breast Asymmetry?

If your radiologist flags a breast asymmetry on your mammogram, additional testing may be needed. The next steps depend on the type of asymmetry and whether there are any other concerning findings.

One of the most common follow-up tests is a diagnostic mammogram. Unlike a routine screening mammogram, a diagnostic mammogram takes more images from different angles to provide a clearer view of the area of concern. Other imaging tests may also be used, such as an ultrasound or MRI.

If you have an asymmetry or a focal asymmetry, you’ll likely be asked to return for a follow-up mammogram in about six months.

If you have a global asymmetry or a developing asymmetry, you may also need a breast biopsy. In this procedure, a sample of tissue is removed from the area of concern. This tissue sample is examined under the microscope to check for abnormal or cancerous cells.

If further testing doesn’t find signs of cancer, you’ll continue to get regular breast cancer screening. Regular screening can help identify developing asymmetries and areas of concern in the early stages.

Managing Breast Asymmetry

Before considering treatment, it’s important to determine the cause of breast asymmetry. If your asymmetry is not linked to a medical condition, there are several options if the difference in breast size or shape bothers you.

Surgery can help create a more balanced appearance. Plastic surgeons commonly perform procedures such as:

• Breast implants
• Breast reduction
• Lipomodelling (fat injections)
• Breast lift

These types of surgeries are usually performed by a plastic surgeon. If you’re considering breast reconstruction surgery after breast cancer treatment, it’s important to talk about your concerns regarding breast asymmetry. If your surgeon is aware of your preferences, they can help you choose the best treatment plan to reach your goals.

Maintaining a stable weight may help prevent fluctuations in breast size, particularly for those who have had radiation therapy. Radiation can cause breast tissue to thicken, which may affect how the treated breast responds to weight changes. Unlike untreated breast tissue, the affected breast may not grow or shrink in the same way as the other breast.

It’s completely normal for adults to have slightly different breast sizes, especially after breast cancer treatment. However, if you notice sudden changes in one breast — such as swelling, shrinking, or a new lump — talk to your healthcare provider as soon as possible.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 77,000 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Do you have breast asymmetry? Did you notice changes in one breast before you were diagnosed with breast cancer? Share your thoughts in the comments below, or start a conversation by posting on your Activities ​​page.