Generally speaking, children can sit at six months of age. For children who cannot sit at this age, it is necessary to see whether the child was born prematurely. If the child is born prematurely, it is necessary to correct the gestational age before evaluating the development of the child, and if the child is not born prematurely, then the problem is not particularly big. If the child is not born prematurely, then the problem is not particularly serious. It is also necessary to see if the child’s other development is within the normal range. If the child is able to hold his head up at three months, and if he is holding his head up steadily at four months, then the child’s gross motor development is basically normal. It is also important to see if the intellectual development and language development are normal. If these developments are normal, then the problem is not particularly big, slowly exercise the child can be, just delayed a month, does not mean that the child’s development is backward, does not mean that the child has a brain injury, because each child’s development is individual differences.