Breast asymmetry, or uneven breasts, refers to when one breast is a different size or shape than the other. Many females have this, and it is not typically a cause for concern. However, substantial asymmetrical breast size or density may be a sign of a higher risk of breast cancer.

Most people’s breasts are slightly different in size, shape, and position. However, uneven breasts or nipples are not usually a cause for concern.

However, breast tissue or breast density that is significantly uneven has links to an increased risk of breast cancer. Regular mammograms can test for abnormalities or changes in breast tissue.

In this article, we look at the causes and diagnosis of asymmetrical breasts, what is typical, and when to speak with a doctor. We also discuss mammogram results relating to asymmetrical breasts and what they mean.

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

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In most cases, breast asymmetry is typical. The two sides of the whole body might be slightly different, though any asymmetry may be more noticeable in the breasts.

However, if a person notices a change in the size or shape of one breast, they need to speak with a doctor to determine the cause.

A significant difference in size or shape between the breasts is less common. This does not always cause medical issues, but a doctor can advise on this.

Uneven breasts are not usually a cause for concern. However, it is essential to have regular mammograms to ensure healthcare professionals can identify breast abnormalities early.

According to 2021 research, women with asymmetrical breast density have an increased risk of developing breast cancer. In fact, their lifetime risk of developing the disease is four to six times higher compared with women of the same age with low breast density.

Any unusual changes in the breast should undergo an examination by a doctor. Changes to look out for include:

  • a lump in or around the breast
  • a lump under the arm
  • tissue that feels thick or firm near the breast or under the arm
  • a change in the size or shape of a breast
  • changes to the nipple, such as it starts to point inward
  • fluid or discharge from the nipple
  • red or discolored, itchy, or scaly skin around the breast
  • dimpled or puckered skin

A mammogram is an X-ray of the breast, which can test for abnormalities, including lumps.

A mammogram might reveal that the breasts have different densities. Doctors call this breast asymmetry or focal asymmetry. Focal asymmetry does not always mean that breasts look or feel any different.

Although dense breast tissue is generally as healthy as less dense breast tissue, a mammogram result may suggest a slightly higher risk of developing breast cancer.

If breast asymmetry is new or changes, healthcare professionals refer to it as developing asymmetry. If a mammogram screening identifies developing asymmetry, there is a 12% risk that the person will develop breast cancer.

Other possible causes for an asymmetrical breast density mammogram result include:

  • a typical variation in the composition of fats and fibrous tissue in the breasts
  • a cyst in one breast
  • fibrosis, or a large amount of fibrous tissue

According to the American Cancer Society, neither fibrosis nor cysts affect a person’s subsequent risk of breast cancer.

During puberty, the left and right breasts often develop at a slightly different pace. Breasts may appear asymmetrical until they have finished growing or may remain in different shapes and sizes throughout a person’s life.

Hormonal changes can cause one or both breasts to change at any point in a person’s life, for example:

Breasts that change size or shape because of hormones often return to their typical size. Hormonal changes can also cause breasts to feel lumpy or lose fat and tissue. However, if these changes do not go away, it is a suitable idea to consult a doctor who can check for any possible health problems.

Some underlying conditions that can affect breast size and shape include:

  • Tubular breasts: Also called breast hypoplasia, tubular breasts can develop in one or both breasts during puberty.
  • Amastia or amazia: A condition that causes problems in the development of breast tissue, the areola, or nipple.
  • Poland syndrome: Where a chest muscle does not develop properly, it can affect the breast on one side of the body.

Anyone who experiences developing asymmetry needs to speak with the doctor for further tests. Tests will likely include another mammogram to check both breasts thoroughly. Ultrasound testing is an additional test to check for any signs of breast cancer.

A doctor may wish to follow up these tests with a biopsy. Tissue from the breast will be sent to a laboratory to check if it contains cancer cells. A healthcare professional will usually take a tissue sample using a fine needle.

Breasts that are slightly different sizes do not pose a higher risk for breast cancer. If the breasts are significantly different sizes, this may increase the risk.

Breast asymmetry is a medical term that refers to breasts that have different densities. This can be a risk factor for breast cancer.

As of 2023, the Preventive Services Task Force recommends that people should start speaking with their doctor about breast cancer screening from 40 years of age. This is instead of the previously recommended starting age of 50 years old.

They encourage those with an average risk to have routine screening from the age of 40–74 years, but screening may start earlier for those with a higher risk.

Factors that increase the risk are:

  • having genetic changes in relevant genes, such as the BRCA genes
  • having a previous history of breast lesions or breast cancer
  • having a history of childhood exposure to radiation in the chest area

Other organizations, such as the American Cancer Society, make different recommendations.

Anyone who experiences unusual changes in the breast should consult a doctor for an examination.

While slight breast asymmetry is typical, a significant difference may warrant medical attention.

Indeed, uneven breast density, identified through mammograms, has associations with an increased risk of breast cancer.

If a person notices a change in the size or shape of one breast, they should consult a doctor to find out the cause.

From the age of 40 years, females should discuss regular breast cancer screening with a doctor.

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