When do babies breathe through their mouths?

Depends on Situation: Infants are obligate nasal breathers as they prefer breathing through their nose rather than mouth. Most infants, however, are able to breathe through their mouth if their nose is blocked. Variation in the length of time until a baby begins oral breathing, but most children and adults are nasal breathers at baseline and mouth breathe to compensate during exercise and colds/stuffy nose for example.