The first year of a baby’s l life is most important. During his or her first year, a baby learns, grows, and changes more rapidly than at any other time. The baby learns to move and manage his or her body. The baby also learns ways of reacting to people and objects in the environment. The main idea is: The changes in the first year of life are most important. A. A baby is happiest during the first year of life. B. A baby’s growth is more rapid during the first year than at any other time.