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 can sit anywhere along the milk line, from the armpit to the groin¹. Some have a small areola, the darker ring, and a few have breast tissue underneath¹. They are often mistaken for moles¹.

Most cases are sporadic. In a small share of families it passes down in a dominant pattern, and some carriers show nothing⁶.

How common it is

The wide range, from 0.2% to 5.6%⁵, reflects real differences between populations⁴. It also reflects how carefully people are examined. A flat extra nipple is easy to mistake for a mole and easy to miss in a quick exam¹.

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 nipple¹. In early modern Europe an extra nipple was sometimes cited as a witch’s mark³.