Why Do Some Women Develop an Axillary Accessory Breast?

Why Do Some Women Develop an Axillary Accessory Breast? Explained by Dr Vikramaditya Salvi, Plastic surgeon on topic of Accessory breast management

Why Do Some Women Develop an Axillary Accessory Breast?

The Fascinating Evolutionary Story Behind Underarm Breast Tissue

"Doctor, why do I have breast tissue under my arm? Is it because I gained weight? Did I do something wrong?"

This is one of the most common questions I hear in my practice.

Many women are surprised to learn that an axillary accessory breast is not caused by exercise, sleeping position, bras, or even weight gain.

In fact, its origins go back millions of years, long before humans walked the Earth.

The answer lies in embryology and evolution.

Nature Gives Us a Clue

If you've ever seen a mother dog feeding her puppies or a cat nursing her kittens, you've probably noticed something interesting.

Unlike humans, most mammals don't have just one pair of breasts.

Instead, they have multiple mammary glands arranged in two parallel rows, extending from the chest all the way towards the groin.

The number varies by species.

Dogs usually have eight to ten nipples.

Cats generally have six to eight.

Pigs may have even more.

Each mammary gland develops along a special embryological structure called the milk line or mammary ridge.

Nursing cat showing multiple mammary glands.

Nature designed this arrangement so animals could feed several offspring simultaneously.

Humans, however, evolved differently.

Humans Still Develop a Milk Line

Although adult humans usually have only one pair of breasts, every human embryo initially develops the same milk line seen in other mammals.

During early fetal development, two thin ridges of tissue appear on either side of the body.

These extend from the armpit (axilla) to the groin.

Historical embryology illustration showing the milk line.

In most people, nearly all of this ridge disappears before birth.

Only a small portion in the chest region remains and develops into the normal breasts.

Sometimes the Milk Line Doesn't Completely Disappear

Occasionally, tiny parts of the milk line persist.

When this happens, additional breast-related tissue may develop along this pathway.

This is known as:

• Accessory breast tissue

• Ectopic breast tissue

• Supernumerary breast tissue

Depending on what remains, different structures may develop.

These include:

• Accessory breast tissue

• An additional nipple (polythelia)

• An areola

• A nipple with an areola

• Small rudimentary breast buds or nubbins

Diagram of the embryological milk line showing possible locations of accessory breast tissue and nipples.)

Why Is the Armpit the Most Common Location?

Although accessory breast tissue can develop anywhere along the milk line, the axilla is by far the most common location.

This is because the normal breast naturally extends towards the armpit through an area known as the axillary tail of Spence.

A small amount of embryonic breast tissue may remain in this region, making it the most likely site for an accessory breast to develop.

This explains why many women notice:

• A soft swelling under the arm

• Fullness while wearing sleeveless clothes

• Pain before menstruation

• Enlargement during pregnancy

• Increased fullness while breastfeeding

The swelling often behaves exactly like normal breast tissue because, biologically, it is breast tissue.

A before and after picture of management of accessory breast management surgery.

Can an Accessory Nipple Occur Elsewhere?

Yes.

An extra nipple may appear anywhere along the milk line.

It may be found on the:

• Chest

• Upper abdomen

• Lower abdomen

• Groin

Many people mistake these for moles or birthmarks and never realise they are actually accessory nipples.

Some remain unnoticed throughout life because they cause no symptoms.

Does Weight Gain Cause an Accessory Breast?

No.

Weight gain can make an existing accessory breast appear larger because breast tissue contains fat.

However, weight gain does not create new accessory breast tissue.

The condition is congenital, meaning it has been present since birth, even if it only becomes noticeable later in life.

Why Does It Become More Noticeable After Puberty or Pregnancy?

Accessory breast tissue responds to hormones in much the same way as normal breast tissue.

Many women first notice it:

• During puberty

• During pregnancy

• While breastfeeding

• With hormonal fluctuations

• Before menstrual periods

This is because the tissue enlarges in response to hormonal changes.

Is It Dangerous?

In most cases, accessory breast tissue is completely benign.

However, because it is true breast tissue, it can develop many of the same conditions that affect the normal breast, including:

• Fibroadenomas

• Breast cysts

• Mastitis

• Rarely, breast cancer

For this reason, any persistent lump or new change should be evaluated by an experienced surgeon.

Why Do Some Women Develop an Axillary Accessory Breast?

The Bottom Line

An axillary accessory breast is not a sign of disease, poor lifestyle, or weight gain.

It is a fascinating reminder of our embryological development and our evolutionary relationship with other mammals.

What appears to be an unusual swelling under the arm is often simply breast tissue that developed along the natural embryological milk line.

For women who experience discomfort, cosmetic concerns, or recurrent swelling, modern surgical treatment can safely remove the accessory breast tissue while preserving a natural contour.

At Cosmetoplast, Wanowrie, Pune, Dr. Vikramaditya Salvi regularly treats women with axillary accessory breast tissue from Pune, Mumbai, across India, and international patients seeking specialised evaluation and treatment.