The baby nine months suddenly a smile mouth crooked may be crooked mouth crying syndrome, brain nerve and facial muscle incoordination, facial paralysis and other causes. 1. Crooked mouth crying syndrome: a congenital malformation caused by a special face, may be by one side of the mouth angle descending muscle is not fully developed, laugh or cry can not be pulled down caused by the special face, the baby nine months suddenly a smile crooked mouth. 2. brain nerve and facial muscles are not coordinated: baby nine months body organs and tissues are still in the development stage, there may be brain nerve and facial muscles are not coordinated phenomenon, caused by a sudden smile crooked mouth situation. 3. Facial paralysis: facial muscles are controlled by the facial nerve, one side of the facial nerve problems, the lesion side of the mouth can not be pulled down, and a normal laugh or cry, may appear crooked mouth. Baby nine months suddenly a smile mouth crooked, 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, under the guidance of the doctor to give targeted treatment or treatment.