The blue birthmark on the baby’s buttocks is called Mongolian spot, because the green buttocks is more common in Mongolian people, so it is called Mongolian spot. When the baby is in embryonic state, the melanocytes move from neural crest to epidermis and fail to penetrate the junction of epidermis and dermis, trapped in the deep dermis to form Mongolian spots, Mongolian spots can appear on any part of the baby’s body, but mostly on the buttocks, waist and back, and a few grow on both lower limbs, trunk, shoulders and other parts, ranging from a few millimeters to dozens of centimeters in size, most of them are greenish blue and do not protrude from the skin. Tactile and normal skin, will gradually fade as the baby grows older, will not bring adverse effects to the baby.