Dogs carry their young from place to place in their mouths. It’s their instinctive behavior when they have to move a juvenile around–which they prefer not to have to do. Pups are much smaller than our human young, and they’re also a lot less vulnerable, and they’ve been configured by Nature to be carried around in the mouth if necessary. It’s terribly unwise to leave any human baby alone in the company of any dog, no matter how beloved or trustworthy, for the first few weeks. If the dog senses a need (the baby is fussy, restless, or needs a fresh diaper) it is likely to want to deal with the perceived problem according to its instincts–carrying the baby to a sheltered place.