Thumb-sucking is a natural reflex for babies. The habit can continue for the first few years of childhood without any permanent damage being done. However, if the habit is not discontinued by the time that the permanent teeth erupt, then the mouth can become permanently deformed. The classic look of a chronic thumbsucker is tiny prominent upper central incisors (or buck teeth) along with a receded chin. It’s possible that there can be a speech impediment also. It’s not really the pressure of the thumb physically pulling the teeth forward that causes the deformity. When the thumb is present when the child swallows, it redirects the tongue downwards away from its natural position in the roof of the mouth. When the thumb is not present, the tongue will continue to thrust forward causing the upper teeth to shift forward and the pressure from the cheeks will collapse the arches inward.