Cyanosis of the infant’s buttocks is a normal phenomenon and is clinically diagnosed as Mongolian spot. Mongolian spot, also known as dermal melanosis and commonly known as fetal spot, is a greenish-gray spot commonly found on the lumbosacral region of infants. The disease is common in Oriental races, especially in Mongolian infants with high incidence. The disease is caused by melanocytes staying in the dermis as they move from the neural ridges to the epidermis during the embryonic period. After birth, children are found to have greenish-gray, grayish-blue, round, oval or irregular patches of varying shades, varying in size, with not very clear margins, which can range from a few millimeters to several centimeters or larger, mostly solitary, occasionally multiple. The most common site is the lumbosacral region, and a few can occur in other areas, such as the ribs and shoulders. If the skin damage occurs on the frontal and temporal areas, it is the same as nevus of Ota. The disease usually fades naturally before the age of 8, and most of them fade within 2 years of age, leaving no traces after fading, and very few can persist into adulthood. Most of this disease can fade on its own and therefore does not require treatment.