Irregularly shaped areas of bluish-black pigmentation found occasionally on the buttocks, lower back or upper arms in newborn infants of African, Chinese and Japanese parentage, and sometimes in the babies of black-haired Europeans. They measure from one to several centimeters in diameter, and usually disappear in a few months. They are sometimes mistaken for bruises.
Bluish-black pigmented marks observed on the lower back and buttocks from birth. These spots are a variation of naevus, resulting from an accumulation of melanocytes (pigment-producing cells). Mongolian blue spots are frequently seen in children of Black or Asian descent. Typically, they fade away by the age of three to four years.