Breast asymmetry, or uneven breasts, refers to when one breast is a different size or shape than the other. Asymmetrical breast size or density may be a sign of a higher risk of breast cancer.

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

However, breast tissue or breast density that is significantly uneven has been linked 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 normal, and when to see a doctor. We also discuss mammogram results related to asymmetrical breasts and what they mean.

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In most cases, breast asymmetry is perfectly normal. In fact, 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 should see a doctor to find out the cause.

A significant difference in size or shape between the breasts is less common. This does not always cause medical problems, 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 that any breast abnormalities are identified early.

Research from 2015 found that women whose breasts vary in size by over 20 percent may be at higher risk of developing breast cancer.

Any unusual changes in the breast should be checked 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, itchy, or scaly skin around the breast
  • dimpled or puckered skin

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

A mammogram might reveal that the breasts have different densities. This is referred to as breast asymmetry or focal asymmetry. Focal asymmetry does not always mean that breasts look or feel any different.

Although dense breast tissue is typically 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, it is called developing asymmetry. If a mammogram screening identifies developing asymmetry, there is a 12.8 percent chance that the person will develop breast cancer.

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

  • normal 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 or cysts affect a person’s subsequent risk of breast cancer.

During puberty, the left and right breast often develop at a slightly different pace. Breasts may appear asymmetrical until they have finished growing, or they may remain 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:

  • at specific points in the menstrual cycle
  • during or near menopause
  • during pregnancy or breast-feeding
  • when using a hormonal contraceptive, such as birth control pills

Breasts that change size or shape because of hormones often return to normal. 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 good idea to visit the doctor, who will 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, which can affect the breast on one side of the body.

Anyone who experiences developing asymmetry should visit 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. The tissue sample is usually removed using a fine needle.

Breasts that are slightly different sizes do not pose a higher risk for breast cancer. If 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.

The American College of Physicians (ACP) recommend that people should start speaking to their doctor about breast cancer screening from 40 years of age.

They encourage those with an average risk to have routine screening from age 50–74 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 see a doctor for an examination.

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