Can babies breathe through their mouth if their nose is stopped up while asleep?

Depends on age/posit: In the first 6m the neck is so short the baby must hold the head up & back to drop the jaw & mouth breathe. This is not a natural position so babies seldom sleep well.The growth of the neck by about 2-3 cm over that time allows the head to be neutral with the jaw open. In early infancy the nose must be clear to breathe without a struggle.