A baby's survival depends on sucking. This skill is so important that babies start practicing long before birth! In utero, it's easy for babies to suck their fingers because the womb's soft walls deflect their hands toward their mouth. After birth, newborns don't suck their fingers much because they have poor muscle coordination.
Sucking's power to calm babies is quite extraordinary: It lowers the heart rate, blood pressure and stress levels; it even reduces crying after shots and blood tests. Sucking on a pacifier at naptime or bedtime is said may help reduce the risk of sudden infant death syndrome (SIDS)