Drooling in three-month-old babies may be a normal phenomenon caused by increased salivary secretion, or it may be caused by mouth ulcers in the baby’s mouth, or it may be caused by diseases such as 21-trisomy syndrome. 1. Physiological factors: three-month-old babies due to the salivary glands began to develop, saliva secretion increased, and the baby can not swallow saliva in time, there will be the symptom of drooling, which is a normal physiological phenomenon. 2. Mouth ulcers: if the baby suffers from mouth ulcers, pain will be produced when swallowing, then the baby dare not swallow saliva, which will also lead to the symptoms of drooling in three-month-old babies. 3. 21-Trisomy Syndrome: 21-Trisomy Syndrome, also known as Down’s Syndrome, is a congenital chromosomal abnormality of the disease, with the disease of the three-month-old baby will also have the symptoms of drooling, will also appear stagnant expression, eyelid fissure is small, the eye spacing is wide, both eyes outside the canthus upward slanting, the bridge of the nose is low and flat, the mouth stretches the tongue, intellectual developmental disorders and other symptoms. After eliminating physiological factors, if the symptoms of drooling in three-month-old infants persist, it is recommended to consult a doctor to find out the cause of drooling and ask the doctor to take care of it.