Sucking the thumb is considered normal behaviour for babies and may even start in the womb. However, according to The American Dental Association, if it continues beyond the age of 3 or 4, it can have a negative effect on developing teeth and jaw. In fact, prolonged thumb sucking can lead to serious dental and speech problems. Research shows that greater than 60% of 10-year-old thumb suckers have a serious misalignment of their teeth.
Persistent thumb sucking can lead to speech problems such as lisping and imprecise pronunciation. Once the habit has been stopped natural improvement of the teeth can occur.