Researchers are still investigating why breast cancer may be slightly more common in the left breast. Theories include right-handed people being better able to detect lumps in their left breast.

If you’ve ever received a diagnosis of breast cancer in your left breast, you’re not alone. Breast cancer seems to occur more often in a person’s left breast than in their right breast, but experts are still working to uncover the reason for this phenomenon.

This article reviews current theories, where in your breast that cancer is more likely to occur, and whether cancer in your left breast is more aggressive than cancer in your right breast.

Is it true that breast cancer is more common in your left breast?

A 2022 study found that breast cancer is slightly more common in a person’s left breast than their right breast.

Researchers examined clinical characteristics from more than 881,000 people with breast cancer with the Surveillance, Epidemiology, and End Results (SEER) program. They observed that left sided breast cancer occurred in 50.8% of the people studied, and right sided breast cancer occurred in 49.2%.

But more research is needed to understand why breast cancer occurs more often in a person’s left breast.

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Researchers aren’t sure why breast cancer seems to occur more often in your left breast. Several unproven theories have cropped up over the years, which we explore below.

Right-handed people are better able to detect lumps in their left breast

In Western countries, approximately 85–90% of people are right-handed. Researchers think that right-handed people may have a better ability to detect breast lumps in their left breast with their dominant hand. And since most people are right-handed, this may account for more diagnoses of left sided breast cancer.

Incomplete breastfeeding

Another right-handed theory centers on incomplete breastfeeding. Nursing mothers may find that it’s easier to hold their baby in their dominant arm, resulting in their left breast being emptied less frequently than their right breast. This might reduce the protective effect of breastfeeding in their left breast. But again, no studies support this theory.

Larger size of the left breast

Breasts aren’t symmetrical, and a person’s left breast is often slightly larger than their right breast. But having more glandular tissue (the part of the breast that makes milk), or more area for cancer to develop, doesn’t mean that cancer is more likely to develop there.

While obesity and dense breasts may increase your risk of breast cancer, there’s currently no evidence that links the size of your breast to an increased risk of breast cancer. In fact, this 2019 study largely disproves any association between breast cancer size and breast cancer risk.

A 2020 review of more than 2.4 million people who received a diagnosis of breast cancer in the United States between 2004 and 2015 found that the most common location for a tumor was the upper outer quadrant of their breast.

This particular review also found that a person’s left and right breast were affected at roughly the same rate but with a slight predominance of their left breast.

Some research suggests that left sided breast cancers are more aggressive than right-sided breast cancers. But research into this topic has been conflicting.

A 2022 study compared outcomes in people with left-breast tumors with those in people with right-breast tumors. The researchers found some differences between left sided tumors and right sided tumors. For example, left sided tumors seemed slightly more resistant to treatment with chemotherapy.

After examining factors such as hormone receptor status and stage of tumors, the researchers suggested that outcomes seemed to be worse in people with left-side breast tumors than in people with right-side breast tumors.

But a smaller 2020 study of one particular ethnic population had different results. Researchers examined data from 228 people who were treated between 1998 and 2020 in Bahrain and found that right sided breast cancers in those people seemed to be more aggressive.

In fact, the researchers found that the people with right-breast cancers had a reduced 5-year survival rate relative to the stage and size of their tumors.

More research is needed to uncover what factors play a role in how aggressive a cancer in your left breast or right breast may be.

Thanks to better awareness and detection of breast cancers, about two-thirds (66%) of people who have received diagnoses of breast cancer are in the early stages (before it has spread beyond the breast) of the disease.

Experts recommend that women follow screening guidelines for early detection. For example, the American Cancer Society recommends that women at average risk between 45–54 years old get a screening mammogram annually — and notes that women between 40–44 years old have the option to do so, too.

Women at high risk of breast cancer are recommended to begin getting annual mammograms and breast MRI much earlier — typically at 30 years of age.

Breast cancers that are detected by a screening exam such as a mammogram tend to be smaller and less likely to have spread beyond your breast. And the earlier a tumor is detected, the more likely treatment will be successful, according to the American Cancer Society.

Language matters

You’ll notice that the language used to share stats and other data points in this article is pretty binary, fluctuating between the use of “female” and “women.” Although we typically avoid language like this, specificity is key when reporting on research participants and findings.

Unfortunately, the studies and surveys referenced in this article didn’t report data on, or include, participants who were transgender, nonbinary, gender nonconforming, genderqueer, agender, or genderless.

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Although current studies have found that breast cancer occurs in your left breast slightly more often than it occurs in your right breast, the evidence regarding why this occurs is still limited.

Also, if you have received a diagnosis of breast cancer in your left breast, it doesn’t necessarily mean that you’ll have a worse outcome than someone with breast cancer in their right breast.

Research into why breast cancer occurs more often in your left breast is still ongoing, as is research into whether left or right sided breast is more aggressive.

Early detection is key to better treatment outcomes. Talk with a doctor about your breast cancer risk and follow their screening recommendations.