Supernumerary nipples

How it works

Early in development, every human embryo has two strips of thickened skin running from each armpit down to the groin. These are the milk lines². In animals with many young, nipples form along the whole line. In humans the line fades except for one spot on each side of the chest. Sometimes a second spot along the line keeps developing, and the result is an extra nipple¹.

Extra nipples are usually small, flat, and slightly darker than the surrounding skin. They sit most often just below the normal nipple, on the chest or upper belly¹. Some have a small areola, the darker ring, and a few have breast tissue underneath¹. Most people who have one assume it is a mole or a birthmark.

Reported prevalence runs from 0.2% to 5.6% of people, depending on the population and on how closely people are examined¹. Men are affected slightly more often than women¹. The trait runs in families in a dominant pattern with variable expression. Family members can carry the gene and show little or nothing¹.

Is it a problem?

Almost never. An extra nipple is a normal variant and needs no treatment. Breast tissue under it can respond to hormones, so it may swell or feel tender during puberty, periods, or pregnancy, in the same way as the normal breast¹. See a doctor if a lump under it grows, changes shape, or leaks. That is the same advice as for any breast tissue.

History and culture

The actor Mark Wahlberg has a third nipple and has spoken about it in interviews³. In early modern Europe an extra nipple was sometimes cited as a witch’s mark¹. Today it is a common enough finding that most doctors see several a year.