The reason why seven-month-old babies are unable to sit still is that at the seven-month-old stage, due to the gradual maturation of various physiological developments, most babies can autonomously raise their heads, lie down, sit and other actions. However, at this stage, many parents will encounter such a problem: The baby is seven months old and cannot sit firmly and falls easily. What is the reason? Reason 1: Insufficient muscle strength cannot support the center of gravity of a seven-month-old baby.