Why Is One Boob Bigger Than the Other? The Science Behind Breast Asymmetry

Boobs come in all shapes and sizes—even two that both belong to the same body.

Photo Credit: Pexels

If you ever looked in the mirror and noticed that one of your breasts looks larger than the other, you’re not alone. Breast asymmetry is a unilateral unevenness of breast dimension, volume or position, by which the breasts are different in size, shape, density or placement.

Breast asymmetry can be congenital or acquired, transient or permanent. Whatever the case, breast asymmetry is incredibly common. In fact, most people with breasts have breast asymmetry to some degree (even if it’s hardly noticeable)—but the reasons range from genetics and hormones to medical conditions that could be cause for concern. According to BreastCancer.org, breast asymmetry is seldom a sign of something serious, and slight breast asymmetry, in and of itself, does not necessarily increase one’s risk of breast cancer. However, significant or sudden variations in breast density could be an indication of a serious health condition like cancer.

Suffice it to say, it is immensely important to take ownership of your breast health by consulting a healthcare professional about perceived changes and, if you’re of the recommended age, have regular mammograms to identify breast abnormalities as early as possible. But it’s also important to educate yourself so you know what to ask your healthcare provider and how to communicate your concerns. That’s why we’ve rounded up some common science-backed reasons why one boob looks bigger than the other—as well as when (if ever!) it could mean more.

1. Genetics

Breasts aren’t always identical, and genetics play an impactful role in determining their degree of symmetry or lack thereof. If your mother, grandmother or other family members have asymmetrical breasts, you’re more likely to have them, too. That’s because breast size, shape, density and positioning are all influenced by inherited traits related to skeletal structure, breast tissue distribution and hormonal sensitivity.

2. Hormones

During puberty, pregnancy, breastfeeding, menopause, menstruation and ovulation, hormonal fluctuations can affect breast size and density. Studies also show that taking hormonal contraceptives or undergoing hormone therapy can cause changes.

During puberty, for example, the breasts can develop at different rates, which means one may grow faster (and larger) than the other. And during pregnancy or breastfeeding, one breast may produce more milk than the other, causing it to be bigger. In fact, one study published in Acta Paediatrica found that nearly three-quarters of mothers (73 percent) reported breast changes after pregnancy. Another study published in the European Journal of Cancer Prevention found that breast size increased by 96 mL, on average, during pregnancy—regardless of the initial volume. Throughout menopause, Surgery (Oxford) research reports that the breasts’ glandular tissue atrophies, the connective tissue becomes less cellular and collagen levels drop, which can shrink the size of the breasts.

Throughout all phases of life, however, estrogen and progesterone (key reproductive hormones) can induce temporary tenderness and swelling by promoting fluid retention and tissue growth. For example, a study published in the British Journal of Radiology found that, at ovulation, breasts are about 5.5 percent smaller than during menstruation. Before menstruation, they are about 8.1 percent larger than at menstruation. And, on average, breast size fluctuates by 76 mL (or about 13.6 percent of the total volume) throughout the menstrual cycle. 

3. Breast tissue distribution

Breasts are made up of fat, glandular tissue and connective tissue. The way this tissue is distributed isn’t always equal, leading to natural differences in size and firmness.

4. Heart location

Interestingly, the position of your heart—which sits slightly to the left side of your chest—could also play a role in breast asymmetry. That’s because the left ribcage may be slightly different in shape to accommodate the heart, affecting how the breast sits. Differences in lymphatic drainage (the system that removes waste and excess fluid) may also lead to size variations.

5. Chest wall and muscle structure

The ribcage and pectoral muscles beneath the breasts can also be asymmetrical, which can cause breast asymmetry. If one side of your chest is slightly more prominent, it can make the breast on that side appear larger because of where and how it sits.

Exercise and strength training, for example, can affect the chest muscles beneath your breasts. If one side of your chest develops more than the other, the pectoral muscle may subtly affect the appearance of the breasts. The breast on the side that is more muscular might appear bigger or more lifted.

6. Habits

Your daily habits can also impact breast size over time. For example, many believe that your sleeping position may affect the shape of your breasts if you consistently sleep on one side and put pressure on one breast (although there is no scientific evidence that suggests this is factual).

Your posture—poor posture, in particular—can also cause one side of your chest to develop differently, change shape due to muscular imbalances or appear to sit higher or lower depending on the evenness of your shoulders. While there is endless research on the effects that breast size and heaviness have on posture, it also makes sense that your posture would affect the appearance of your breasts. 

7. Medical conditions

While most cases of breast asymmetry are completely normal, some rare conditions can cause more noticeable differences in the size, shape, density or position of the breasts.

• Poland Syndrome is a rare condition in which one side of the chest has underdeveloped muscles, affecting breast size.

• Juvenile Hypertrophy is a condition in which one breast grows significantly larger than the other during puberty.

• Atypical Ductal Hyperplasia (ADH) affects the breast tissue cells because the milk ducts experience excessive growth.

• Scoliosis, the abnormal curvature of the spine, can impact breast asymmetry and the position of each breast.

• Diabetic Mastopathy, a rare complication of diabetes associated with long-term use of insulin can cause hard, painless masses to form in the breasts, altering their appearance.

• Amastia or Amazia cause developmental issues of the breast tissue, areola or nipples.

• Cysts or fibroids—which are benign lumps—can make the breasts appear differently sized or shaped.

• Trauma or surgery from breast-related injuries or procedures (such as augmentation) can affect the appearance of breasts.

  
  

Should you be concerned about asymmetrical breasts?

You should definitely see a doctor if you experience the following, which could be indicators of underlying conditions that need medical attention:

• Significant or sudden changes in breast size, shape, density or position

• Lumps in or around the breasts and underarms

• Pain or tenderness

• Changes to the nipple, including nipple direction or discharge

• Skin changes, including redness, dimpling or puckering

While differences in breast size, shape, density or position do not necessarily denote cancer and research is still underway regarding the relationship between asymmetric breasts and cancer risk, one study published in Sage Journals found that a breast-volume asymmetry ratio of over 20 percent was significantly higher in the cancer group studied. Another study published in Breast Cancer Research has found that “breast asymmetry was greater in healthy women who later developed breast cancer than in women who did not.” And more recent research published in the Austin Journal of Women’s Health found that women with asymmetrical breast density are at an increased risk of developing breast cancer. 

But it’s not only breast cancer to keep in mind. Another study published in RadioGraphics found that about 20 percent of people with lymphoma (a type of blood cancer) develop changes in breast symmetry.

Again, in most cases, breast asymmetry is not a cause for concern. However, you should consult your healthcare provider if you are worried about the differences between your breasts—both because of serious health scares or simply because it’s taking a toll on your body confidence.

Your healthcare provider may prescribe a mammogram, which can observe four different types of asymmetry:

1. Asymmetry - When dense breast tissue is visible in one view of the mammogram.

2. Focal asymmetry - When dense breast tissue is visible in two views of the mammogram.

3. Global asymmetry - When a large area of dense breast tissue is visible in two views of the mammogram.

4. Developing asymmetry - When asymmetry is observed in a mammogram that wasn’t present in previous imaging.

   
   

Only a healthcare provider can help you determine whether or not mammogram results are cause for concern and consult you about next steps (like continuing screening), if necessary.

How to balance breast asymmetry

If breast asymmetry bothers you because of the way the breasts look alone, there are a few ways to create a more even look. Of course, surgical breast augmentation is one way to go, but studies show that the recurrence of asymmetry is a common weak point when it comes to breast-recontouring procedures. 

With that in mind, here are some non-surgical ways to balance breast asymmetry if it is hurting your self-confidence or causing you any kind of psychological distress: 

• Choose bras with removable pads to adjust the appearance of volume on one side or the other.

• Silicone or foam inserts can also help even out the look of your breasts’ volume.

• Bras with adjustable straps can help lift the breasts to change their positioning.

• Fashion tape and compression garments can both help adjust the positioning of the breasts for a more even look under clothing.

• The use of posture corrects can help you maintain better posture, which can, in turn, help to improve the appearance of symmetry with proper alignment.

• Chest exercises can improve symmetry, especially if you focus on the side that seems smaller.

• Regular massages can improve blood circulation and lymphatic drainage, which may slightly enhance the size of the smaller breast over time, as well as reduce any swelling and fluid retention in the bigger breast.

  
  

The bottom line

Breast asymmetry is common and occurs for many reasons that are not necessarily a reason to worry. Unless you experience significant or sudden changes, breast asymmetry is rarely a cause for concern. Whether you choose to embrace or enhance your natural shape, remember that perfect symmetry is more of a myth than a reality for most humans with boobs. It is totally normal and natural to have different-sized breasts!