A supernumerary nipple is, quite simply, an extra nipple. In humans, it takes the form of a third nipple, although mammals that naturally have more than two nipples can also develop extras. As a general rule, these nipples are entirely harmless, and no medical action needs to be taken to deal with them. Some people have them removed for aesthetic reasons, however.
These additional nipples appear along the “milk lines,” which are areas of the body where the skin is thicker, allowing the development of breast tissue and nipples to occur. Supernumerary nipples can appear anywhere from the head down to the feet, and they may vary considerably in composition. Some, for example, are fully formed nipples with a small amount of breast tissue, capable of potentially producing milk, while others are much more rudimentary. In some cases, one may simply be a little tuft of hair.
Often, these nipples are mistaken for moles, and people are surprised to learn what they actually are. In humans, they are sometimes referred to as thripples, third nipples, accessory nipples, or polymastia. They arise during fetal development, when something along the milk lines goes slightly awry, triggering the development of an additional nipple. Around one in 18 humans has an extra nipple, making the condition not at all unusual.
Although people most commonly think of a third nipple when they hear of extra nipples, it is possible to develop multiple supernumerary nipples. The extra nipples are often visible at birth, and they change little over the course of a person's life, usually not developing significant breast tissue in puberty. Fully-formed extra nipples can potentially be at risk of breast cancer, however, and in people who are diagnosed with breast cancer, they may be removed, just to be on the safe side.
Removal of a supernumerary nipple is usually a relatively simple task, since the area is small. Some stitches may be taken or a skin graft may be applied, and the area is typically sore for a week or so while the early stages of healing begin. A small scar may be left behind by the removal, especially if the nipple was large and especially well-formed.
Some people actually enjoy their extra nipples, feeling that they confer a bit of a special status. Like other unusual markings that appear in the course of fetal development, these growths have been viewed with superstition historically. In some cultures, having a third nipple was supposed to be lucky, while in other instances, it was viewed as a sign of witchcraft or heresy.