Polymastia is a congenital condition in which a person has one or more extra breasts in addition to the typical pair. It develops before birth when breast tissue that normally disappears during embryonic development instead persists, and it affects an estimated 1% to 6% of the population depending on the study and how broadly the condition is defined. Most people with polymastia never realize they have it until hormonal changes like puberty, pregnancy, or menstrual cycles cause the extra tissue to become noticeable.
Why Extra Breast Tissue Develops
Early in embryonic development, a ridge of tissue called the “milk line” forms on each side of the body, running from the armpit down through the chest and abdomen to the groin. In most mammals, multiple breasts develop along this line. In humans, most of the milk line regresses, leaving only the two typical breasts on the chest. Polymastia happens when part of the milk line fails to regress completely, leaving behind extra breast tissue that develops alongside the normal breasts.
About 67% of the time, this extra tissue appears somewhere along that original milk line path on the torso. But it can also show up in less expected places, including the back, thigh, or even the face. The tissue forms during the same developmental window as the normal breasts, which is why it contains the same types of glandular cells and responds to the same hormones.
What Polymastia Looks and Feels Like
The condition exists on a spectrum. In its most complete form (sometimes called a “third breast”), the extra tissue includes a nipple, an areola, and functioning glandular tissue underneath. But many cases are far subtler. Some people have only a small patch of glandular tissue under the skin with no visible nipple, while others have a small extra nipple with no breast tissue behind it (a related condition called polythelia). A Finnish researcher named Kajava created a classification system with eight categories ranging from a complete extra breast down to just a patch of hair marking where breast tissue would have formed.
The most common location for accessory breast tissue is the armpit (axilla). In this spot, it often feels like a soft, slightly firm mass that has the same consistency as normal breast tissue. Many people assume it’s just a pocket of fat, and the condition is frequently misdiagnosed as a lipoma, a swollen lymph node, or a cyst. Imaging with ultrasound or mammography can distinguish accessory breast tissue from these other possibilities.
Hormonal Symptoms and Cyclical Changes
Accessory breast tissue is hormonally responsive, meaning it reacts to the same hormonal shifts that affect your regular breasts. During the luteal phase of the menstrual cycle (roughly the two weeks before a period), rising estrogen and progesterone levels stimulate the glandular cells, which can cause the extra tissue to swell and become tender. Some people report cyclical pain in the armpit area for two to three weeks before each period without ever connecting it to breast tissue.
Pregnancy and breastfeeding can make polymastia dramatically more obvious. Elevated prolactin levels during lactation can trigger milk production from the ectopic tissue. In one documented case, a woman experienced active lactation from a mass in her armpit while nursing her second child, along with daily discomfort in both axillary regions that had worsened over five years of cyclical hormonal changes. For many people, pregnancy is the first time they discover they have accessory breast tissue at all.
During puberty, the tissue may enlarge alongside normal breast development. After menopause, it typically shrinks as hormone levels decline, just like regular breast tissue does.
Cancer Risk in Accessory Breast Tissue
Because accessory breast tissue contains the same glandular cells as normal breasts, it carries the same potential for developing breast cancer. The incidence of cancer arising specifically in accessory tissue is low, estimated at around 0.2% to 0.6% of all breast cancers. The most common type found in ectopic tissue is invasive ductal carcinoma, which accounts for 50% to 75% of these cases.
The challenge is that lumps in accessory tissue, particularly in the armpit, are easy to overlook or attribute to something benign like a cyst. If you have a known area of accessory breast tissue, it’s worth being aware that the same screening principles that apply to your regular breasts apply to that tissue as well. Any new lump, change in size, or persistent pain in the area deserves evaluation.
When and How It’s Treated
Many people with polymastia never need treatment. If the tissue is small, painless, and not cosmetically bothersome, monitoring it over time is a reasonable approach. Treatment is typically considered when the tissue causes chronic cyclical pain, interferes with movement (especially in the armpit), produces unwanted lactation, or creates cosmetic concerns.
The standard surgical approach for accessory breast tissue in the armpit combines liposuction with removal of the glandular tissue through a small incision. Some surgeons also remove overlying skin, but recent practice increasingly favors leaving the skin intact initially. Removing skin creates longer, more visible scars and can sometimes restrict arm movement or lead to tight, stretched scarring. Instead, some surgeons recommend waiting at least a year after the gland is removed to see how the skin naturally drapes and contracts before deciding whether a smaller secondary procedure is needed.
When the extra tissue includes a visible nipple and areola, skin removal is more likely to be part of the initial procedure for aesthetic reasons. Recovery from the surgery generally takes one to two weeks for most people to return to daily activities. Swelling, tenderness, and some numbness or tingling around the incision site are common in the first few weeks and gradually resolve over the following months. During the early recovery period, you’ll want to avoid heavy lifting or overhead movements to reduce swelling and protect the surgical site.
Getting the Right Diagnosis
One of the biggest practical issues with polymastia is that it’s frequently mistaken for something else. A soft mass in the armpit or along the torso is commonly assumed to be a lipoma (a benign fatty lump), an inflamed sweat gland, a follicular cyst, or swollen lymph nodes. The key distinguishing feature is that accessory breast tissue tends to change with your menstrual cycle, swelling and becoming tender before a period and then settling down afterward. Lipomas and cysts don’t follow that pattern.
If physical examination alone isn’t conclusive, ultrasound or mammography can identify the characteristic glandular structure of breast tissue. In some cases, a small biopsy confirms the diagnosis. Getting the correct diagnosis matters not just for peace of mind but also because accessory breast tissue needs to be monitored differently than a lipoma, given its small but real potential for developing the same conditions that affect normal breasts.