Medically known as polythelia, or supernumerary nipples, an extra nipple is a common congenital skin variation. These structures are present from birth but often go unnoticed until they become physically altered. The sudden visibility of what appears to be a “third nipple” in adulthood is usually the result of underlying physiological changes acting upon a structure that was already there. Understanding this phenomenon requires looking at the origins and development of this extra tissue throughout a lifetime.
What Accessory Nipples Are
An accessory nipple represents a minor anomaly resulting from the incomplete regression of the embryonic mammary ridge, often called the “milk line.” During gestation, this ridge forms as a thickened strip extending from the armpit down to the groin. While most of this tissue disappears, a small remnant can persist anywhere along this line, resulting in a supernumerary nipple. These accessory nipples are common, found in up to 6% of the population, and vary widely in structure. They can range from a full nipple and areola complex to a simple patch of pigmented skin or hair.
Developmental Timeline
Supernumerary nipples are congenital, but they are often so small and underdeveloped that they are overlooked at birth. In infancy, they may resemble a tiny mole, freckle, or pigmented spot, making them difficult to distinguish from other skin lesions. They frequently remain dormant throughout childhood, only becoming noticeable later when hormonal activity increases.
The most common periods for these latent structures to become apparent are during major hormonal shifts. Puberty, with its surge in sex hormones, can cause an accessory nipple to enlarge, darken, or become tender. For women, a previously unobserved accessory nipple may first become apparent during pregnancy, as high hormone levels cause pigmentation and possible swelling.
Sudden Visibility in Adulthood
Latent accessory tissue does not truly appear for the first time in adulthood, but it can become visible or more prominent due to adult-onset physiological changes. Accessory nipples, especially those with underlying glandular tissue (polymastia), are fully responsive to hormonal fluctuations. Certain medications or endocrine disorders can trigger these changes, even outside of pregnancy or the menstrual cycle.
Adult-onset hormonal shifts caused by specific medications can lead to the enlargement of latent mammary tissue. Drugs that affect the balance of estrogen and testosterone or increase prolactin levels, such as spironolactone or some antipsychotics, may cause tenderness and swelling. Endocrine disorders like polycystic ovarian syndrome (PCOS), which involves androgen excess, can similarly influence the appearance and size of these structures.
Significant changes in body weight can also affect the visibility of a previously hidden supernumerary nipple. A marked weight gain may cause the accumulation of fat tissue beneath a shallow accessory nipple, potentially pushing it outward and making it more noticeable. Conversely, rapid weight loss can alter skin tension, causing a small, mole-like polythelia to become more defined or prominent.
Mimics and Differential Diagnosis
If a new bump or pigmented area appears on the torso that resembles a nipple, it is important to remember that many common skin conditions can mimic the appearance of a supernumerary nipple. Growths often mistaken for accessory nipples include benign lesions such as skin tags, pigmented moles (nevi), and dermatofibromas. These lesions can sometimes be located along the milk line, further complicating self-diagnosis. Other, less common growths, such as neurofibromas or soft tissue tumors, may also present as a lump or papule that could be confused with accessory tissue. Because of the possibility of these mimics, or the rare chance of malignant changes, any new or changing skin lesion requires professional evaluation. A medical professional can accurately distinguish between a benign accessory nipple and a condition requiring further investigation.