Ten months of the baby’s mouth is always open during the day may be upper respiratory tract infection, oral diseases, adenoid hypertrophy and other causes. 1. upper respiratory tract infection: ten months of the baby resistance is weak, if there is no good warmth measures, prone to upper respiratory tract infection, which leads to nasal congestion, cough and other symptoms, the baby breathing is not smooth, there will be the daytime mouth always open. 2. Oral diseases: such as mouth ulcers, the baby will feel the pain in the mouth, in order to relieve the pain, may appear during the day the mouth is always open. 3. adenoid hypertrophy: if the baby has rhinitis and other diseases, failed to cure in time, may cause adenoid hyperplasia and hypertrophy, resulting in the baby can not normally breathe through the nose, thus appearing during the day the mouth is always open. Ten months of the baby’s mouth is always open during the day, there may be other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, give targeted treatment or standardized treatment.