Accessory Breast Tissue in the Armpit: Why the Lump That Swells Before Your Period Is Not Fat, How a Third Nipple and an Extra Breast Form Along the Same Embryonic Line, Why an Ultrasound Comes Before Any Cannula, What Excision Does That Liposuction Cannot, and What the Tissue's Cancer Risk Means for Leaving It Alone

Accessory breast tissue is the most common thing in an armpit that patients are told is something else. The story is remarkably consistent. A woman notices, usually in her late teens or twenties and often for the first time during a pregnancy, a soft fullness at the front of one or both armpits that shows in a sleeveless top, aches in the week before her period, and does not shrink when the rest of her does. A primary care physician calls it fat. A trainer calls it fat. A med spa offers to melt it. A liposuction practice quotes a price for it. And the tissue, which is not fat but a gland that happens to be sitting in the wrong place, goes on doing what glands do, cycling and swelling and occasionally developing the diseases that glands develop, while everyone around it treats it as a contouring problem.

This piece is about that tissue specifically: what it is, how it got there, and what should happen before and during any attempt to remove it. It is the companion to the earlier piece on the bra roll and back liposuction, which mentioned accessory breast tissue in a paragraph as the thing a surgeon has to rule out before suctioning the side of the chest. That paragraph deserves an article, because the distinction it draws is the difference between a cosmetic procedure and a diagnostic one, and because the number of patients who receive the wrong one is not small.

The milk line: why breast tissue ends up in an armpit, a groin, or on a thigh

The short answer: every human embryo grows two ridges of breast-forming tissue running from the armpit to the groin, the ridges are supposed to regress everywhere except the chest, and when a segment fails to regress it becomes accessory breast tissue, which appears in the armpit in the large majority of cases and can range from a full extra breast with a nipple to a patch of gland under normal skin.

Between roughly the fourth and sixth week of gestation, two thickened bands of ectoderm appear on the front of the embryo. Anatomists call them the mammary ridges or milk lines, and in a four-legged mammal they are the reason a dog or a pig has a row of teats from chest to belly. In humans the ridges run from the axilla, through the chest, down the abdomen, and into the inguinal region on each side, and by about the ninth week they have disappeared everywhere except a single point over the fourth intercostal space on each side, where the breast develops. The disappearance is a programmed event, like the loss of the webbing between fingers, and it is not always complete. When a portion of the ridge persists, the tissue it would have become persists with it, and the result is breast tissue outside the breast.

The Finnish surgeon Yrjö Kajava classified the possibilities in 1915, and his eight classes are still the ones textbooks and pathology reports use:

• Class I, a complete accessory breast: glandular tissue with a nipple and an areola. This is polymastia in its fullest form, and in the armpit it can lactate.
• Class II, gland and nipple, no areola.
• Class III, gland and areola, no nipple.
• Class IV, glandular tissue alone, under skin that looks entirely ordinary. This is the form most often mistaken for fat, because there is nothing on the surface to announce it.
• Class V, nipple and areola without gland, sometimes called pseudomamma.
• Class VI, a nipple alone. This is polythelia, the supernumerary or third nipple, and it is by far the most common form of the whole family.
• Class VII, an areola alone.
• Class VIII, a patch of hair alone, where a nipple would have been.

The prevalence figures are broad because the studies are old, the populations differ, and a small third nipple on a lower rib is easy to record as a mole. Polythelia is usually quoted at somewhere between one and five percent of the population, appears in men and women at similar rates, most often sits just below the breast along the line the ridge followed, and runs in some families with a pattern that looks autosomal dominant with incomplete penetrance. Glandular accessory tissue, the form this article is mostly about, is quoted at roughly two to six percent of women, and in something like two thirds of those cases it is in the axilla, because the upper end of the ridge is the segment most likely to persist. It is bilateral in about a third of patients, more often on the left than the right for reasons nobody has explained, and it is present at birth in every case even though it is almost never noticed until puberty makes it grow.

That last point matters for how patients understand their own history. Accessory breast tissue does not appear. It is revealed, by the same hormonal events that develop the breasts proper: puberty, the menstrual cycle, pregnancy, lactation, hormonal contraception, and weight gain, which enlarges the fat around the gland and makes the whole region more prominent. A woman who says the lump appeared during her first pregnancy is describing the moment an accessory gland that had been there since before she was born received the signal to prepare for milk.

How to tell accessory breast tissue from armpit fat, a lipoma, and a lymph node before anyone books an operating room

The short answer: accessory breast tissue is firmer than fat, changes size with the cycle and with pregnancy, sits at the front of the axilla in the line between the breast and the armpit crease, and is confirmed by ultrasound, which shows the same fibroglandular pattern as the breast; no one should be treating an axillary lump of any kind without an image of it.

The examination begins with the history, because the tissue gives itself away in the calendar before it does in the hand. Three questions separate the gland from the fat that can surround it. Does the fullness get larger, firmer, or tender in the week before a period and settle afterward? Fat does not have a menstrual cycle. Did it enlarge noticeably during pregnancy or breastfeeding, and did it ever leak? A fat pad does not engorge, and a fat pad has never produced milk. Has it stayed the same size through a meaningful weight change? Axillary fat rises and falls with the rest of the body; a gland embedded in it does not.

On examination the tissue is usually a discrete or semi-discrete mass at the anterior axillary fold, the ridge of tissue between the outer edge of the breast and the front of the armpit. It is firmer and more rubbery than the fat around it, sometimes lobulated in the way breast tissue is lobulated, and it can be tender to compression in a way fat is not. Skin changes over it, a small raised dot that has been called a mole, a faint darker disc, or a tuft of hair in an unusual place, point toward one of the higher Kajava classes. Both arms are examined raised and lowered, because the tissue is often visible only with the arm elevated, which is exactly the position in which a patient sees it in a mirror.

The differential is worth listing, because it is the list a surgeon should be able to recite and a patient should expect to hear:

• An axillary fat pad. Soft, diffuse, without a discrete edge, unchanged through the cycle, and continuous with the fat of the chest wall and the back. This is the tissue the bra roll piece is about, and it is a liposuction problem.
• The axillary tail of Spence. Normal breast tissue that extends from the upper outer breast toward the axilla as a tongue, continuous with the breast and not separate from it. It is prominent in some women, more so after weight gain or breastfeeding, and it is the tissue the piece on gender-affirming top surgery describes as the lateral chest roll that a chest surgery has to address. Accessory tissue, by contrast, is a separate island with normal fat between it and the breast, and ultrasound tells them apart.
• A lipoma. A benign fat tumor, soft, mobile, slow growing, and without any relationship to the cycle. Common in the axilla and easy to confuse on examination alone.
• A lymph node. The axilla holds twenty to forty of them. An enlarged node is usually deeper and more posterior than accessory tissue, firmer, and sometimes attached to what is around it. A node that is hard, fixed, or growing is a node that needs a diagnosis, not a contouring plan.
• Hidradenitis suppurativa. Recurrent inflamed nodules and abscesses in the sweat-gland-bearing skin of the armpit, often with tunnels and scarring. It can coexist with accessory breast tissue and complicate its removal.
• A cyst or an abscess. Either can arise in the skin of the axilla or within accessory glandular tissue itself, and an abscess in accessory tissue during lactation is a real presentation.
• A vascular malformation or a hernia of breast tissue through a defect in the fascia, both uncommon.

Imaging resolves the list, and the order is not a matter of taste. The American College of Radiology's appropriateness criteria for a palpable mass in the breast or axilla place ultrasound first in women under thirty and combine diagnostic mammography with ultrasound in women forty and over, with the thirties handled by ultrasound first and mammography added when the ultrasound is anything other than clearly benign. On ultrasound, accessory breast tissue looks like breast tissue: a region of mixed echogenicity with the fibrous and glandular pattern a radiologist sees every day in the breast proper, sitting in a place where it should not be, separate from the breast and from any node. Fat looks like fat. A lipoma looks like a well-defined fatty mass. A node has a cortex and a fatty hilum. When the ultrasound is equivocal, when the patient is over forty, or when anything about the mass is suspicious, mammography with an axillary view or magnetic resonance imaging follows, on the same logic set out in the piece on breast imaging for the breast itself.

The point of insisting on imaging is not caution for its own sake. It is that the two most common treatments offered for an armpit bulge, liposuction and an injectable fat-dissolving drug, are both treatments for fat, and neither has any business being applied to a mass that has not been shown to be fat. A practice that quotes for either without an ultrasound on file has skipped the step that determines whether the treatment can work at all, and the patient who accepts the quote has agreed to have a gland suctioned or injected on the assumption that it is not one.

What can go wrong inside tissue that was never supposed to be there

The short answer: accessory breast tissue is real breast tissue and develops the full range of breast disease, from cyclical pain and fibroadenomas to mastitis, abscess, and carcinoma, and because it sits outside the field of a screening mammogram and outside most clinicians' mental map, disease in it tends to be found later than the same disease in the breast.

The reason this matters for a cosmetic decision is that the tissue does not know it is in the wrong place. Under a microscope, accessory breast tissue is lobules, ducts, and stroma, responsive to estrogen and progesterone and prolactin exactly as the breast is, and everything the breast can do, it can do. The common problems are the benign ones. Cyclical mastalgia, the premenstrual ache, is the presenting symptom in a large share of patients, and in the axilla it is aggravated by the friction of the arm, the bra strap, and the seam of a sleeve in a way breast pain is not. Fibrocystic change and simple cysts occur. Fibroadenomas, the firm benign lumps of young women's breasts, occur. During pregnancy the tissue enlarges, sometimes dramatically, and after delivery it can engorge to the point of visible swelling and real pain. Where there is an accessory nipple, milk can appear from the armpit. Where there is glandular tissue without an outlet, which is the common situation, the milk has nowhere to go, and the result can be a galactocele, a milk-filled cyst, or a mastitis that progresses to abscess. Lactating axillary accessory tissue is one of the more distressing presentations a new mother brings to a physician, and the correct answer is usually not surgery but time, because the tissue involutes after weaning as the breast does; the surgical timing question is taken up below.

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Accessory breast tissue does not know it is in the wrong place. It cycles, it engorges, it lactates, and it develops every disease the breast develops, in a spot no screening mammogram images and most clinicians never think to examine.

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The uncommon problem is the one that changes the calculus. Carcinoma arising in accessory breast tissue is rare, accounting for something on the order of three to six of every thousand breast cancers in the published series, and the axilla is the most common site because the axilla is the most common site for the tissue. The cancers are histologically the same cancers that arise in the breast, invasive ductal carcinoma most often, and they behave the same way. What differs is when they are found. Several series report ectopic breast cancers presenting at a more advanced stage than breast cancers of the same period, and the explanations are not mysterious. A screening mammogram is positioned to image the breast, and while the oblique view captures the lower axilla, tissue high in the armpit lies outside the field. An axillary lump in a woman who has been told for a decade that she has armpit fat is not a lump she reports promptly. And a clinician who does not have accessory breast tissue on the list does not order the ultrasound that would find the mass inside it. None of this means a woman with accessory tissue is at high risk; the tissue is a small fraction of her total breast tissue and the cancer rate is proportionate to that. It means that the tissue is a place where disease can hide, and that whatever she decides about removal, she should know it is there, her physicians should know it is there, and it should be examined as part of her breast examination for the rest of her life.

There is one more association worth addressing because patients will encounter it online. Beginning in the 1970s and 1980s, a series of small studies, several from Hungary and Israel, reported that children with supernumerary nipples had a higher rate of kidney and urinary tract malformations than children without, and for a time some pediatric texts recommended a renal ultrasound for any child with a third nipple. Larger studies that followed, including a substantial American series published in 2001, found no such association in their populations, and the consensus in the United States is that an isolated supernumerary nipple in an otherwise healthy child does not call for renal imaging. The association may be real in some populations and absent in others, or it may have been an artifact of small numbers. An adult reading about it before a cosmetic excision can set it aside; a parent whose child has a third nipple and any other unexplained finding should raise it with a pediatrician rather than with a plastic surgeon.

Excision versus liposuction: why the cannula that flattens a bra roll leaves glandular tissue behind

The short answer: glandular accessory tissue is fibrous and dense and does not pass through a liposuction cannula, so the operation that removes it is a direct excision through an incision hidden in the axillary crease, with liposuction reserved for the fat around the gland, and every specimen goes to pathology.

The single most useful thing a patient can understand about the operation is the physical difference between fat and gland. Subcutaneous fat is lobules held in a loose fibrous net, and a cannula moving through it breaks the lobules loose and carries them out under suction, which is why liposuction in its various forms works at all. Glandular breast tissue is a different material: dense, fibrous, rubbery, organized around ducts, and anchored to the surrounding fascia. A cannula pushed into it either bounces off or tunnels through it, removing the fat around and within it and leaving the gland itself in place, smaller only to the extent that its fatty component was reduced. Surgeons who treat gynecomastia know this well, and the piece on gynecomastia surgery describes the same problem in the male chest: suction takes the fat, and the firm disc of gland under the nipple has to be cut out through an incision. The accessory gland in the axilla is that disc, without the nipple to mark it.

Ultrasound-assisted and power-assisted liposuction are sometimes marketed as able to handle glandular tissue, and it is true that ultrasonic energy emulsifies fibrous fat more effectively than a plain cannula. It does not turn a gland into something a cannula can remove, and the published experience of liposuction alone for accessory axillary breast tissue is small, short in follow-up, and notable for recurrences of cyclical swelling when residual gland regrows or re-engorges. There is also a diagnostic cost that patients are rarely told about. Suctioned tissue arrives at pathology as fragments in a canister, if it is sent at all, and a pathologist cannot examine architecture in fragments. An excised specimen arrives whole, oriented, and readable, and the reasoning laid out in why every specimen belongs in a pathology report applies with more than the usual force to a tissue that can harbor carcinoma. An accessory breast should be removed in a way that lets someone look at it.

The excision itself is a well-described operation. With the patient marked standing and with the arm raised and lowered, because the tissue moves and the mark must correspond to where it is on the table, the surgeon makes an incision in or parallel to the axillary skin crease, where the scar will sit in a natural fold and be hidden with the arm down. Through that incision the glandular tissue is identified, separated from the surrounding fat, and removed as a single specimen down to the fascia over the chest wall muscles. Where a nipple or areola is present, an ellipse of skin containing it is removed with the gland, and the incision is designed so that ellipse is the incision. Where the overlying skin is redundant, as it often is in a patient whose accessory tissue enlarged through pregnancies and then involuted, a crescent of skin is removed to prevent an empty fold; where it is not, the skin is left alone. Liposuction is then used, if at all, to feather the fat at the edges of the excision so the removed gland does not leave a step or a hollow. A small drain is sometimes placed for a day or two on the reasoning the piece on surgical drains sets out, because the axilla is a lymphatic crossroads and the dead space left by a removed gland collects fluid. The procedure takes roughly an hour per side and is done under local anesthesia with sedation or a light general on the considerations described in how anesthesia choice shapes the operative plan.

Removal of a supernumerary nipple alone, Kajava class VI, is a smaller matter and is often performed by a dermatologist. It is an elliptical excision of the nipple with a rim of skin, closed in a line oriented along the skin tension lines, under local anesthesia in the office in twenty to thirty minutes. Two points distinguish it from removal of a mole, which is what many patients believe they are having done. First, it goes to pathology, because the thing being removed is breast tissue and the report should say so; the reasoning in the piece on cosmetic mole removal about shave versus full-thickness excision applies, and for a supernumerary nipple the answer is always full thickness, because a shave leaves the ductal tissue beneath. Second, the surgeon should examine the surrounding area for glandular tissue under the nipple, because a class VI lesion on the surface can be a class II lesion underneath, and a nipple excised over a gland that is left behind is a nipple that will be followed by a lump.

Recovery, complications specific to the axilla, cost, and who should be doing the operation

The short answer: recovery from axillary excision is short but the armpit is a moving, sweating, lymphatic-rich fold that seromas, stretches scars, and hosts a sensory nerve, the operation should wait until at least several months after weaning and until adolescence is complete, it costs in the range of a small body contouring case, and it belongs to a surgeon who will image first and send tissue to pathology afterward.

The recovery follows the anatomy. The incision sits in a joint that moves through a wide arc many times an hour, so the first two weeks involve keeping the arm below shoulder level, avoiding reaching and lifting, and wearing a compression garment or a snug sports bra that holds the axilla closed, for the reasons the evidence on compression garments supports in any dead-space operation. Showering is permitted early on the schedule most practices use for small incisions. Ordinary desk work resumes within days, driving when the arm can be raised comfortably, and the return to exercise follows the general pattern for when exercise is actually safe, with overhead and pulling movements last, at roughly four to six weeks. Swelling and firmness at the site persist for two to three months and the final contour is a six month judgment.

The complications are the complications of the axilla, and a candid consultation names them:

• Seroma. The axilla drains a great deal of lymph and an excision leaves a cavity where it can pool. Seroma is the most common complication of the operation, managed as the piece on seroma describes, and it is the reason for the drain and the compression.
• Hematoma and wound separation. A moving incision in a fold that sweats is at higher risk of both than an incision on the abdomen, and the pattern in the piece on wound dehiscence describes why an armpit incision that opens is usually one that was stretched too soon.
• Numbness of the inner upper arm. The intercostobrachial nerve, a sensory branch that crosses the axilla to supply the skin of the inner arm, runs through the field, and stretching or dividing it produces the numbness familiar to any patient who has had axillary lymph node surgery. Most of it recovers over months on the timetable in the piece on numbness after plastic surgery; some does not.
• Scar. A scar in the axillary crease is well hidden with the arm down and visible with it raised, and axillary skin scars unpredictably: hypertrophic and occasionally keloid in patients prone to either, on the reasoning in the scar care piece. A patient with a history of poor scarring should see the surgeon's axillary scars at a year, not at three months.
• Contour problems. Over-resection of fat around the gland leaves a hollow that is more conspicuous than the fullness it replaced. Under-resection of the gland leaves the cyclical swelling in place and the patient back where she started, with a scar.
• Residual tissue. Accessory glandular tissue does not always have a crisp boundary, and small islands left behind can re-engorge in a subsequent pregnancy. Complete excision at the first operation is the point of doing it as an excision.
• Flare of hidradenitis. In a patient with hidradenitis suppurativa, surgery in the axilla can provoke it, and the excision is planned with a dermatologist's input.

Timing is the question most often gotten wrong. The tissue should not be excised during pregnancy or lactation, when it is engorged, vascular, and likely to leak milk into the wound, and the sensible interval is at least three to six months after weaning, when involution is complete and the true volume of gland and skin can be assessed, on the same logic the piece on mommy makeover timing sets out for the breast. A woman planning further pregnancies can reasonably have symptomatic tissue removed between them, because the tissue does not come back once excised, but should understand that any residual gland will announce itself in the next pregnancy. In adolescents the tissue can appear alarmingly during puberty and settle as development completes, and excision should wait until breast development is finished, with the framework in the piece on cosmetic surgery for teenagers governing the rest. And the examination for surgical planning should be done at two points in the cycle if the surgeon is at all uncertain about the boundary between gland and fat, because a mass examined premenstrually and a mass examined mid-cycle are not the same size.

Cost is structured as any surgical quote is structured, in surgeon, facility, and anesthesia fees, and in Los Angeles a bilateral axillary excision under sedation is commonly quoted somewhere in the low to middle four figures per side, with a single-side excision under local at the lower end and a case that includes skin excision and liposuction of the surrounding chest at the upper end. A supernumerary nipple excision in a dermatology or plastic surgery office is quoted in the hundreds to low four figures. Insurance is the one place where accessory breast tissue differs from most of the procedures on this site. Because the tissue is a congenital anomaly that produces documented symptoms, pain, recurrent mastitis, restriction of arm movement, skin irritation, some plans will cover its excision as reconstructive when the symptoms are documented over time and imaging is on file, on the coverage logic described in the piece on insurance and medical necessity. Removal for appearance alone is cosmetic and is not covered, and a practice that offers to code a purely cosmetic excision as symptomatic is offering to commit fraud on the patient's behalf.

Who should do the operation is a narrower question than it looks. The excision is not technically difficult, and a board-certified plastic surgeon or a breast surgeon does it comfortably; the credentialing framework in the piece on board certification applies as it does to everything. The discriminating question is not skill but sequence. The right surgeon orders or reviews the ultrasound before proposing anything, distinguishes the gland from the fat around it on examination and on the image, plans an excision rather than a suction for the gland, sends the specimen to pathology as routine, and tells the patient that she has breast tissue in her armpit and that the breast examination she receives for the rest of her life should include the site. A practice that offers to treat the bulge without having established what it is, whether with a cannula, an ultrasound probe, or a syringe of deoxycholic acid, has demonstrated the one thing that should disqualify it.

The honest summary

• It is a gland, not fat. Accessory breast tissue is breast tissue left behind along the embryonic milk line, present in roughly two to six percent of women, most often in the armpit, present from birth and revealed by puberty, the cycle, and pregnancy. A fullness that swells before a period or engorged during breastfeeding is not an axillary fat pad, and no amount of dieting or suction changes what it is.
• Imaging comes first, and the order is not optional. Ultrasound distinguishes gland from fat, lipoma, the axillary tail of the breast, and a lymph node; mammography and MRI follow when the patient is over forty or the ultrasound is anything other than clearly benign. A practice that quotes liposuction or a fat-dissolving injection for an armpit lump without an image has skipped the step that decides whether the treatment can work.
• The tissue develops breast disease. Cyclical pain, cysts, fibroadenomas, lactational engorgement, mastitis, and, rarely, carcinoma, on the order of three to six per thousand breast cancers, which tends to present later because the site is outside the mammogram and outside most clinicians' thinking. Whatever is decided about removal, the site belongs in every breast examination for life.
• Excision removes it; liposuction does not. Glandular tissue is too dense and fibrous for a cannula. The operation is a direct excision through the axillary crease, with skin removed when a nipple is present or the skin is redundant, liposuction reserved for feathering the fat at the edges, and the whole specimen sent to pathology. A third nipple is a full-thickness excision, not a shave, and the surgeon checks beneath it for gland.
• Time it and staff it correctly. Not during pregnancy or lactation; at least three to six months after weaning; after breast development is complete in adolescents. Expect seroma as the leading complication, temporary numbness of the inner arm, a crease scar that shows with the arm raised, and a six month contour. Symptomatic tissue can qualify for insurance coverage with documentation; cosmetic removal does not, and a practice that offers to code around that is offering fraud.

Accessory breast tissue is one of the few problems in aesthetic surgery where the cosmetic complaint and the medical one are the same piece of tissue, and where getting the diagnosis right is worth more than getting the contour right. A patient who walks into a liposuction consultation with an axillary bulge and walks out with an ultrasound order has been served well, even if the practice made nothing on the visit. A patient who walks out with a suction date has been sold a treatment for a condition she does not have, and will discover in her next pregnancy, or in a mammogram that does not reach high enough, that the thing she paid to have removed is still there. The tissue was left behind by an embryo that did not finish a job. The surgeon's job is to finish it properly, or to explain clearly why, for this patient, it is better left alone and watched.