Oregon Department of Wildlife provide the following overview: 

Fawns and other young animals are rarely orphaned, so please leave them where you find them. Until they are strong enough to keep up with their mothers, fawns are left alone while their mother goes off to feed. Mother deer will stay away from the fawns to avoid leading predators to their young. Does return at dawn and dusk to feed and/or move their young.  The babies know to stay still and quiet, tucked into the grass where their mother left them.