Many individuals notice a soft, fleshy area under their armpit and wonder about its nature. This often leads to questions about whether it is simply fat or something else. This article clarifies what this area might be.
Understanding Accessory Breast Tissue
The “fat under your armpit” is accessory breast tissue, also known as supernumerary breast tissue or polymastia. This tissue is breast glandular tissue that develops outside the typical breast area. It commonly appears along the “milk line,” an embryonic ridge extending from the armpit down to the groin. While it may resemble a fatty lump, it differs from regular adipose (fat) tissue because it contains glandular components.
Accessory breast tissue behaves similarly to normal breast tissue. It can swell, become tender, or change in size and sensitivity during hormonal fluctuations. These changes are often noticed during the menstrual cycle, pregnancy, or lactation. This tissue affects 2% to 6% of women and 1% to 3% of men.
How and Why It Forms
Accessory breast tissue originates during fetal development, around the fourth to sixth week of gestation. At this stage, thickened bands of ectoderm, called mammary ridges or “milk lines,” form along the body, extending from the armpits to the inner thighs. Normally, only one pair of these ridges persists to develop into breasts in the chest region, with the remaining tissue regressing.
Accessory breast tissue arises from incomplete regression or displacement of mammary ridges during embryogenesis. This occurs when remnants of the embryonic milk line persist and develop into additional breast tissue outside the typical chest area. While congenital, its prominence can be influenced by hormonal changes throughout life. Puberty, pregnancy, and breastfeeding involve significant hormonal shifts that can cause existing glandular tissue, including accessory tissue, to enlarge. Weight gain can also make accessory breast tissue more noticeable, as both the glandular tissue and surrounding fat cells may increase in size.
Addressing Related Concerns
Accessory breast tissue is a common, benign variation. However, because it contains breast glandular tissue, it is susceptible to the same conditions that can affect normal breasts. These can include fibrocystic changes or lumps like fibroadenomas. Rarely, it can also develop breast cancer, though this accounts for a small percentage of all breast cancer cases (0.2% to 0.6%).
Seek medical advice if the lump changes in size or texture, becomes painful, or if there is any discharge. Diagnostic methods include a physical examination and imaging techniques like ultrasound or mammogram to differentiate accessory breast tissue from other lumps, such as lymph nodes or simple fat deposits. Management options include observation if asymptomatic. For cosmetic concerns, discomfort, or medical issues, surgical removal through liposuction or excision may be considered.