【英语时差8,16】背后的蓝色胎记

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Parents of East Asian, East African and Native American descent are not usually surprised when their babies are born with distinctive¹ 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 spine² 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. But as the Mongols merged³ with other populations, the trait became less pervasive⁴, and in some populations it is now undetectable. 

 

Although uncommon⁵ in Caucasian and other groups, a similar spot can be seen microscopically⁶ in all newborns regardless of race. The birthmark is produced in a deep layer of the skin by a large cluster of melanocytes--or pigment⁷ cells--formed from precursor⁸ cells that migrated into the skin during embryonic⁹ development. 

 

Sometimes the birthmarks are confused with bruises¹¹ because the light scattering¹² effect responsible for making a bruise¹⁰ 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. And 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 prodded¹³ or slapped by a spirit to leave "pre-life" and be born, others say it's a sign of royalty¹⁴. In truth, they are congenital birthmarks.  

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