Premium quality foods should always be fed, but with that being said...........a pregnant bitch should be switched to Puppy Food approx 3 weeks prior to whelping, and continue with the puppy food until all pups are weaned. I have always kept my bitches on puppy food until the puppies go to their new homes at 9 weeks of age. Then she is switched back to adult food. I figure the extra 4 weeks of puppy food for her (after the pups are weaned) will help get her back into condition. My last litter (3 years ago), the puppies were born at the beginning of June.....then in Sept, she was back in the show ring. You could NOT tell she had had puppies, except her nipples were a little larger.
Several meals per day after whelping also, as the mother needs extra food. A nursing bitch can consume between 3-5 times the NORMAL amount of food.