Everything You Need to Know About the Third Nipple

Ah, the mysterious third nipple. Why do some people have one, and how do they form? At the BBC, Jason Goldman reports that while some have considered the "supernumerary nipple" a fault of the evolutionary process, similar to wisdom teeth or the tailbone, recent research shows that extra-nipple formation may be far more involved. Here are key takeaways:

• Researchers are finding a correlation between gender, ethnicity, geographic region, and nipples. For instance, one study found 0.6% of white American infants have a third nipple, while their black American counterparts clock in at 1.63%; Israeli infants had an even higher rate of 2.5%.
• The supernumerary nipple is very unusual: Only an estimated 200,000 people are born in the US with what the National Institute of Health calls a "rare disease."
• Humans can and have had up to six additional nipples, though more than three is considered exceedingly rare.
• The nipples (both normal and supernumerary) form during the fourth week of embryo fertilization. Two strips of tissue across the chest (called "mammary ridges") regress in-utero, creating two nipples, but sometimes they don't regress completely, leaving an extra nipple.
• Some supernumerary nipples can contain breast tissue, and undergo the same hormonal changes as normal nipples.
• Though rare, supernumerary nipples exist in places other than the chest. One such "ectopic supernumerary nipple" was reportedly found on a French woman's thigh in 1827. It produced milk and "her infant ... took it willingly," writes Norman Grossl in the Southern Medical Journal.

Click for Goldman's full story. (More human body stories.)