Thumb, Finger or Pacifier Sucking

All healthy newborns start life and sustain it with an
urge to suck. Embryos have been observed sucking
their thumbs while in the womb. Sucking is one of
a baby’s inherent reflexes that is an essential ability
for basic survival—if it were not present, the infant
would not seek food or nourishment.

For many infants, the sucking instinct is not satisfied
by feedings alone. Non-nutritive sucking, that is sucking thumbs, fingers, pacifiers and other objects, is a
healthy normal behavior and offers young children
a feeling of security, comfort, pleasure and relaxation
during the first few years of life. This habit helps children to cope with different situations and emotions.
Virtually all young children at one time or another
place their fingers, fist, pacifier, thumb, or other
objects in their mouth to suck. As children grow and
develop, most naturally discontinue this habit.

https://cchp.ucsf.edu/sites/g/files/tkssra181/f/thumbsen1105_adr.pdf