Small-month-old infants sleeping with their mouths open may be caused by nasal disfluency, so you can use moist cotton swabs to clear nasal secretions and seek medical attention for checkups and treatment if necessary. Normally, infants sleep with their mouths closed. When infants sleep with their mouths open to breathe, it may be caused by nasal insufficiency. For example, when infants have allergic rhinitis, upper respiratory tract infections, swollen nasal mucosa, nasal polyps, nasal cysts and other diseases, blocking the nasal cavity, which can make infants’ noses breathe imperfectly and turn to breathe through their mouths. Parents can use a cotton swab moistened with saline, gently inserted into the nasal cavity, rotate the swab, clean up the secretions in the nasal cavity, to alleviate the symptoms, generally not recommended to take special medication. It is recommended to bring the baby to the hospital in time for examination, to clarify the specific causes, and then under the guidance of the doctor for targeted treatment.