Bubbles in the mouth of a three-month-old baby are usually due to physiological causes or pathological causes such as bronchitis and pneumonia. 1. Physiological reasons: usually due to the baby salivary glands began to secrete exuberant, but the swallowing function is not yet fully developed, which can lead to the baby can not be secreted out of a large number of saliva swallowed into the mouth, appearing spitting bubbles. Parents do not need to worry too much, as the baby grows older, the symptoms of spitting bubbles can be significantly reduced. 2. Pathological causes: If the baby spits out bubbles, accompanied by coughing or increased respiratory rate, may be combined with respiratory infections such as bronchitis, pneumonia, etc. The bubbles are usually sticky and thick. These bubbles are usually sticky and may affect the appetite, causing a loss of appetite. It is recommended to bring your baby to the hospital in time to check whether there is inflammation in the lungs, and under the guidance of specialists, timely treatment according to the specific circumstances.