A extra nipple, in either men or women, is not rare. Occurring in up to 5% of the population, it is often mistaken for a mole due to its circular appearance. What separates it from a mole is that it is raised and has a textured and not a smooth appearance. It does not usually grow in size and only gets bigger in proportion to the body as it grows.
The location of a supernumerary nipple is also a giveaway as it will lie along the milk lines. The milk lines are embryonic vertical lines that extend from the armpit to the legs from which arise breast tissue and nipples. They appear early in embryonic life by the end of the second month in utero and well before sexual identity is formed. (hence why men have nipples even though they will never breastfeed) The nipples will form along this line and humans always have two nipples…although more are capable of forming along the milk lines.
In most cases of extra nipples, medically known as polythelia, it will just be a single one and is usually located below the breast or chest. Hence the term third or triple nipple. In more rare cases it will be more than just a nipple and it may develop into an actual breast mound albeit much smaller than a natural breast. (polymastia)
Given the relative common occurrence of an accessory nipple, it would not be rare to find it on a prospective breast augmentation. This is an example of a 35 year-old female who came in for breast implants and wanted to have a raised mole removed at the same time. Its appearance was clearly that of a nipple and not a typical mole. When removed at the time of surgery, its appearance can be seen to be similar to a nipple when laid up against the augmented breast.
Dr. Barry Eppley