If you're traveling with a newborn, Delta and Delta Connection® Carrier flights do need an approval letter from a physician saying it's all right for the newborn to fly. The following infant age restrictions apply to all Delta and Connection Carrier flights:
Airline Infant Age Requirement or Restriction
Delta Less than 7 days Physician travel approval letter
Comair® Less than 12 days Physician travel approval letter
SkyWest® Less than 7 days Physician travel approval letter