It is normal for newborns to smile.
Newborns do not smile and usually only appear to be either crying or sleeping. Starting around the time of full term, newborns will experience a sudden and instantaneous smile during sleep. This smile is an instinctive response or a subconscious smile.
The structural morphology of the sulci and gyrus within the newborn’s brain is not clearly developed, the neural tissue of the brain is not yet well developed, and myelination on the outside of the nerve fibers has not yet formed. In newborns with high neural excitability, it is easy to produce passive motor generalization responses and elicit strong physiological reflex actions.
As a result, some newborns may show uncontrollable movements such as body tremor, shaking, sudden and rapid facial twitching, or involuntary body movements and hand and foot dances. A newborn’s inexplicable smile can be considered a manifestation of facial “tremor”.