The blue color on the baby’s back is the result of pigment cell accumulation during embryonic development, commonly known as birthmark, mainly for the following reasons: 1, when the embryo grows to 4-5 months, the spot pigment cells will gradually appear, usually on the baby’s buttocks and back, its shape is round or irregular shape, generally no special treatment, gradually fade with age, the baby’s body will not be affected. The birthmark is not higher than the surface of the skin, does not hurt or itch when pressed by hand, and some of the skin appears bruised, and may be caused by extrusion during the birth process, which will generally be slowly absorbed and fade over time; 3, It is recommended to take the baby to the hospital for routine blood tests and blood clotting.