What is an underbite? An underbite is a malocclusion where the lower teeth extend outward farther than the upper front teeth. This can create a bulldog-like appearance in the mouth and face. Some cases can be severe causing the lower teeth to extend far forward. Other cases are milder and almost unnoticeable. What causes an underbite? · Thumb sucking. · Tongue thrust when swallowing. · Using a pacifier in children above age 3. · Genetics. · Injury. · Tumors. Why does an underbite need to be fixed? Patients with milder cases may not have any o...