Occasional yelling in a 9 month old baby is considered normal, if it occurs frequently it is considered to be a cause of disease, such as epilepsy, cerebral palsy, etc. is not normal. 1. Normal: Babies are too young to express their emotions and will express them by yelling, for example, if the baby is angry or if the baby is very happy, yelling will occur. In addition, if the parents often yell at the baby, or if there are people around them who yell a lot, the baby will imitate others and appear this way. 2. Unusual situations: (1) Epilepsy: If a 9-month-old baby’s yelling is a sudden and unexplained seizure, it may also be epilepsy, which is a chronic brain disease caused by abnormal neuron discharges in the brain. There are more types of epilepsy, and some of them will show up as yelling. (2) Cerebral palsy: Cerebral palsy leads to motor dysfunction and is accompanied by mental retardation, sensory and perceptual abnormalities, mental behavioral abnormalities, etc. Due to mental retardation, the thinking of the child is not the same as that of a normal child, and the way of expression may be strange, which may be manifested as yelling and screaming, and there are usually backwardness in the development of locomotor movement, reduction of active movement, and abnormalities in the muscle tone. For normal phenomenon does not need special treatment, if it is the cause of disease, it is recommended to treat as soon as possible.