The scientific name for your ex's third nipple is a supernumerary nipple. According to the NIH, supernumerary nipples are apparently quite common. Exact numbers vary but it's estimated about 1% of the population have them.
Usually people don't have more than one extra nipple, but one man had seven total nipples. Most peoples' just look like little moles, some are clearly nipples and some even lactate during pregnancy!
Typically, extra nipples are found along the milk lines, also known as mammary lines, which run from the armpits to the inner thighs. In early human embryonic stages, clusters of cells clump together along the milk lines.
For most people, two of these clumps will become breasts. Usually any extra clumps regress but sometimes they don't and they become extra nipples, areolas and even full breasts. While most extra nipples exist along these lines, about 5% are found elsewhere, and have a different name: ectopic supernumerary nipples.
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