An underbite is when the lower teeth close in front of the upper teeth. An underbite is also referred to as a class III malocclusion.
According to Dr. Thomas C. Field, an orthodontist for more than 30 years, an underbite can be an imbalance of the jaws, which could (in some cases) lead to a severe imbalance that would require surgery for correction.
In other cases, the imbalance is less severe and orthodontic treatment can correct or accommodate the imbalance.
Dr. Field also states that since the underbite is often a growth imbalance, it can become more pronounced as the patient grows.
Source:
Thomas C. Field D.D.S. 23 July 2008.

