When puppies are born, they don't eat solid foods. They live off the milk their mothers produce. Mother dogs naturally produce milk, which her puppies consume by suckling, as human babies do with their mothers. If a mother doesn't get the nutrition she needs, she may not be able to provide her young with enough to eat.
A dog's pregnancy lasts about two months. When her pups are born, they're hungry and ready to eat. They start suckling almost immediately, so the mother starts producing milk before they're born. This may be as early as a week before she goes into labor or as late as the day before. If your dog is pregnant, talk to your veterinarian about increasing her meal portions both before and after giving birth. Typically, during gestation and nursing, you'll feed more food and different food -- puppy meal or high-energy-dog meal. She needs extra food so her body can take care of itself while producing milk for her pups.