I have recently had a baby girl, and I have noticed that she has an indentation under her left nipple. I have heard that people can have a third nipple. It doesn't really look like her other nipples, there is no roundness or areola. It is positioned under her left nipple. Are additional nipples common? How will I know if it is an additional nipple?
While I can't tell you whether the indentation under your daughter's nipple is a third nipple without actually examining her, I can tell you that it is entirely possible to be born with a "third nipple" (also know as a "supernumerary" or "accessory" nipple). While accessory nipples are not really common, they are common enough (on the order of 1 to 5 percent of the population) that we do see them. They most often involve just a small areola (darker skin) around a nipple, but on occasion can also include some underlying breast tissue. While this doesn't seem to fit with your description, the location of your daughter's indentation may be in line with the typical location of supernumerary nipples, which occur most often right under the breast, but can be anywhere along an (imaginary) line drawn from the breast to the groin.
While third nipples are usually harmless and don't cause any symptoms, it's worth mentioning that if they are identified in infants with any other physical abnormalities they can signify the need for a kidney evaluation. It's therefore important to bring your daughter's indentation to the attention of her pediatrician, who will be able to do a physical exam and reassure you if what she has is actually a third nipple or something else, as well as let you know whether it warrants any further evaluation.