If a baby has a green buttock, if it is present at birth as a single round, oval or square, light blue, dark blue or brown spot, it is considered to be a Mongolian spot. Mongolian spots are caused by melanocytes, which originate from the neural ridge. During the embryonic period, melanocytes gradually move towards the epidermis, and these melanocytes stay in the dermis if they fail to cross the junction of the epidermis and the dermis, so Mongolian spots will appear at birth. Mongolian spots have no effect on health, and they can subside by themselves without special treatment, after a few years, they can subside by themselves, so there is no need to be overly nervous and worried.