Before she becomes pregnant and then the first couple weeks of her pregnancy she should be fed a premium adult dog food.
Then starting at week four or five of her pregnancy start adding premium performance dog food or even a premium puppy food, check the label and be sure it is not large breed puppy formula which is generally lower in fat, protein and minerals.
About half way through her pregnancy increase your pregnant dogs daily meals to 3.
Also keep in mind during the last week your pregnant dog may need to be fed every 3-4 hours because the growing puppies are taking up more room.
The last couple weeks of the pregnant dogs gestation is when the most fetal growth takes place.
Keep An Eye On Her Weight
After whelping (giving birth) and the first 3 to 4 weeks of lactating she may be eating 2-4 times more than ate before she became pregnant.
Watch her weight, she should be gaining at a healthy level and not allow her to become obese, a mistake made with many pregnant dogs.
Your goal here is to have her weigh the same at weaning as she did prior to becoming pregnant, of course this depends on whether she was at her ideal weight at that time.
She should continue to have regular, but not strenuous, exercise to help her maintain her muscle tone and not become overweight.