Thumb sucking is ordinary in infancy and resolves on its own for most children. Its effect on the developing jaws depends almost entirely on how long it continues.
Light forces sustained over time reshape bone
Bone responds to pressure by remodelling, which is the same principle orthodontic appliances rely on. The force required is small provided it is applied for long enough each day.
A thumb resting between the teeth for hours, particularly during sleep, easily exceeds the duration threshold at which remodelling begins.
Brief or intermittent sucking does not accumulate that duration, which is why occasional daytime habits behave very differently from persistent nocturnal ones.
The palate narrows because of the cheeks
Sucking requires negative pressure inside the mouth, and it also positions the tongue low and away from the roof of the mouth.
The tongue normally rests against the palate and provides an outward force that balances the inward pressure of the cheek muscles during growth.
With the tongue displaced downward, the cheeks act unopposed and the upper arch narrows, which can produce a high vaulted palate and a crossbite at the back teeth.
Front teeth move where the thumb sits
The thumb occupies vertical space between the upper and lower incisors, preventing them from meeting as they erupt and leaving an opening in the bite.
Pressure from the back of the thumb tends to push the upper incisors forward while the lower ones are pushed back, increasing the horizontal gap between them.
Both effects are positional rather than structural at first, which is why they often improve once the habit stops early enough.
Timing determines whether it self-corrects
Habits that cease before the permanent incisors erupt frequently resolve without intervention, since the teeth erupt into a bite no longer being interfered with.
Once permanent teeth have erupted into the altered position and the habit persists, spontaneous correction becomes less likely and orthodontic assessment is usually suggested.
The transverse narrowing of the palate is the change least likely to self-correct, because it involves the width of the bone rather than tooth position alone.
How the habit is usually approached
Most approaches begin with the least intrusive measures, since the habit often serves a self-soothing purpose and pressure alone tends to be counterproductive.
Appliances that interrupt the habit exist and are placed by a dentist when simpler measures have not worked and the effects are progressing.
A dentist or orthodontist is the right person to judge whether changes seen so far need active management or observation, since the answer depends on the child's stage of development.