Four-month-old babies with bad breath may be caused by physiological factors, oral diseases, gastrointestinal diseases and other factors. 1. Physiological factors: four-month-old babies just mild bad breath, no other clinical symptoms, may be a normal physiological phenomenon, because four-month-old babies can not brush their teeth, there may be milk residue in the mouth, there will be mild bad breath. 2. Oral disease: parents in the daily care of improper, so that the baby oral disease, such as mouth ulcers, will lead to the baby’s mouth odor, may also be accompanied by more than crying, feeding difficulties and other symptoms. 3. gastrointestinal diseases: for example, indigestion, gastrointestinal dysfunction, etc., may lead to bad breath, may also be accompanied by bloating, diarrhea, nausea, vomiting and other symptoms. Four-month-old babies have bad breath, there may be other reasons, it is recommended to go to the hospital in time to improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.