Why Are My Breasts Uneven?

Asymmetrical breasts are common and not usually cause for concern.

Most breasts are not created equal. Even on the same person, two breasts are likely to vary slightly in size and shape. Asymmetrical breasts are perfectly normal, and all breasts will fluctuate in size over the course of a woman's menstrual cycle, especially during ovulation and during her period. 

Breasts are usually fuller and more sensitive during ovulation and then shrink back to their normal shapes and sizes during menstruation. It's most likely that the difference in the size of your breasts is due to normal growth variations. 

There are other natural causes that lead to differences in breast size. For example, hormone changes during puberty can cause one breast to start growing before the other.

When Should I Worry About Uneven Breasts?

Have you been conducting regular breast self-exams? You may find a breast lump that is causing one breast to look larger than the other. These lumps are growths of tissue that develop within your breast. They can be cancerous, but many are simply caused by a variety of benign conditions.

Sometimes, breast lumps are actually cysts, (fluid-filled sacs). These are also generally noncancerous.

Breast lumps can also be caused by fibrocystic breasts, a condition that causes a woman's breasts to feel lumpy or rope-like, which is relatively common. This condition usually isn't any cause for concern but, if you experience persistent or worsening breast pain, you should talk to your healthcare provider.

Finally, breast lumps can be caused by fibroadenoma, non-cancerous breast tumors that vary in size and shape. These can be surgically removed by a breast surgeon should they fail to shrink over time.

When should you be concerned about breast cancer? If your breasts have always been uneven, you probably don't have to worry. But if you've noticed a sudden change in the size, shape, and/or appearance of your breasts later in life, it could be a sign of a medical condition.

Medical Conditions Affecting Breast Size

Breasts may appear uneven during pregnancy when hormonal changes and the body's preparation for breastfeeding make breasts larger and sometimes lopsided. During breastfeeding, breasts can appear to vary in size, especially if the baby favors nursing on one side.

Atypical ductal hyperplasia, or excessive growth of tissue in the breast's milk ducts, is considered a precancerous condition. In this condition, there are more cells lining the duct than normal and some of these cells are irregular in shape and size. This can lead to benign lumps in the breast, which may affect its appearance. 

Hypoplastic breasts, also called underdeveloped breasts, may be small, thin, spaced far apart, or very uneven. The areola may appear very large. It's a condition that one is born with, and as the person grows, the breast tissue doesn't fully develop. 

How to Determine the Cause of Breast Asymmetry

If necessary, your healthcare provider may recommend a mammogram, a breast ultrasound, or a breast biopsy. There is no evidence that breast asymmetry is an individual risk factor for breast cancer. However, it is important for women to know what is normal for their breasts' appearance and to report any changes to their doctor.


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