When the human body is in embryonic state, melanocytes fail to penetrate the junction of epidermis and dermis when moving from neural crest to epidermis, resulting in melanocytes trapped in the deep dermis, thus forming Mongolian spots. Mongolian spot is a kind of birthmark, which is usually found on the buttocks, waist and back, with light grayish-blue or dark blue color, mainly round, oval or square shape.