Thumb sucking is a general habit among young kids. Children often suck their thumb or finger because of their natural digging and sucking reflex. Children who continue this pattern in childhood usually stop between the ages of two and four.

Thumb sucking is usually not a problem in early childhood. However, as the child gets older, this habit can become a problem.  You can also know more about how to stop thumb sucking via https://www.amazon.co.uk/Age-2-7-Stop-Thumb-Sucking/dp/B00VQU2DZS.

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Thumb sucking past the toddler age can pose some risks to the health of the teeth and oral cavity. This risk increases the longer the child sucks the thumb.

Overbite And Underbite

One of the most common consequences of prolonged thumb sucking is developing an overbite or underbite. 

Overbite is a condition in which the upper teeth reach further than the lower teeth. An underbite is the opposite of overbite and occurs when the lower teeth protrude above the maxillary teeth.

The two conditions create differences that can cause discomfort or difficulty chewing. The intensity of the thumb in the mouth can put unnecessary pressure on the teeth, pushing them out, and causing over-biting or underbite.

Speech Obstruction

Thumb-sucking has the potential to change the palate and affect the development of the palate and teeth. These changes can influence your child's ability to speak properly. 

It is not uncommon for certain language difficulties, such as whispering or the inability to pronounce loud consonants such as "D" or "T", causing long-term social problems.