If a child’s buttocks and back are bruised, if they are not present at birth and appear for a short period of time, and if the child cries and resists when pressed, consider the petechiae caused by bleeding due to trauma, which requires prompt treatment. If the child has such phenomenon at birth, it is considered to be formed by pigmentation. During the development of the fetus, melanocytes will move from the neural ridge to the epidermis, and if they cannot penetrate the junction of the epidermis and dermis smoothly, they will be trapped in the deep dermis, thus forming a blue color on the buttocks and back, resembling petechiae. Most of them will gradually fade away with age until they disappear completely without leaving any trace.