Parents of East Asian, East African, and Native American descent are not usually surprised when their babies are born with distinctive blueish-gray or blueish-green birthmarks on their backside.

Eighty to ninety-five percent of infants from these ethnicities bear the marks, and they are also quite common on Hispanic babies. The spots appear at the base of a baby's spine at birth or a few weeks afterwards, and usually fade after a few years.

The birthmark is often called a "Mongol Spot" because in ancient times the trait was characteristic in Mongolian tribes. However, as the Mongols merged with other populations, the trait became less pervasive, and in some populations it is now undetectable.

Although uncommon in Caucasian and other groups, a similar spot can be seen microscopically in all newborns regardless of race. The birthmark is produced in a deep layer of the skin by a large cluster of melanocytes, or pigment cells formed from precursor cells that migrated into the skin during embryonic development.

Sometimes the birthmarks are confused with bruises, because the light scattering effect responsible for making a bruise look blue is the same reason why Mongol Spots look blue. The pigment for Mongol Spots is deep within the skin, so longer waved light, which appears as the color red, is absorbed. Shorter waved light that appears as blue, is reflected back to our eyes.

Some cultures believe the spots are left from where the baby was prodded or slapped by a spirit to leave "pre-life" and be born, others say it's a sign of royalty. In truth, they are congenital birthmarks.