http://news.bbc.co.uk/1/hi/health/4202466.stm
When an embryo is developing, the formation of organs is tightly controlled by specific genes.
This process controls the development of two breasts in humans, but sometimes something goes wrong and leads to fewer, extra or misplaced breasts and nipples.
One in 18 people has an extra nipple, which can look like freckles or moles.