Mouth breathing is often caused by a partially blocked airway, usually because of an allergy or enlarged adenoids or tonsils. A doctor should evaluate these conditions. Frequent mouth breathing can cause dry, red, swollen gums. This can be especially noticeable around erupting baby and permanent teeth.
In children younger than 8, about half do some breathing through their mouths, presumably not due to a medical problem. Most children outgrow this habit by the age of 8.
The relationship between ongoing (chronic) mouth breathing and malocclusion (“poor bite”) is unclear, but the two are often seen together. The most common trait of people who chronically breathe through their mouths is an elongated (longer) lower face and a narrowed upper arch in the mouth (maxillary constriction). Cheek muscles pressing in on the upper side teeth cause these traits. Experts question whether mouth breathing is responsible for these skeletal and dental changes.