Preterm babies sleeping with their mouths open may be caused by oversized tongues, inaccessible noses and other reasons. 1. Tongue is too big: preterm babies sleep with their mouths open, mainly caused by their own tongue is too big, but generally this situation is not very likely to happen, the reason for this situation is that the preterm babies in the lying down, the tongue will continue to stretch back, which leads to the throat is obstructed, resulting in sleeping with their mouths open. 2. Nose: Preterm babies sleeping with their mouths open can also be caused by a blocked nose, such as a cold or a blocked nose, but if the nostrils have not been cleaned for a long time, it is more likely that the mouth will be open during sleep. Preterm babies with open mouths in their sleep should be seen immediately, the cause of the problem should be clarified under the guidance of the doctor, and standardized treatment should be carried out in cooperation with the doctor.