Vital bodily functions like breathing, heartbeat, digestion and temperature control are regulated unconsciously. They are overturned by what is known as the autonomic nervous system. Since these processes have to be kept going non-stop, walking or sleeping, it would not make sense for them to be controlled voluntarily. It is only because they are fully automatic that important reflexes occur quickly and reliably.
The automatic stepping movements, which babies make when held upright with feet touching a flat surface, are also lost after three months. Only much later are the leg muscles and sense of balance sufficiently developed to enable the child to stand upright and walk. Other babyhood reflexes, such as swallowing, coughing and sneezing, are retained permanently.