I have been raising Yorkies for over 12 years. I feel that it is important to not over supplement. If your females is eating well then just a good all stages of life dog food is all the pregnant momma needs. Some people will switch the momma to a puppy food the last two weeks of her pregnancy. Since I feed an 'all stages' dog food I don't find that necessary.
It is thought by many that giving the momma a calcium supplement prior to her delivering puppies will make her more prone to getting eclampsia after the puppies are born. Eclampsia can be deadly for the momma if not treated immediately. That is why it is very important to keep the Cal-sorb on hand. Please read up on eclampsia in dogs:
Eclampsia (Puerperal Tetany, Milk Fever, Hypocalcemia) in Dogs
Some females can get eclampsia prior to having puppies so you have to be prepared in case that occurs. I have never had a female get pre-eclampsia. Osteo-Form is a chewable calcium-phosporus supplement for dogs. I am not going to say not to give it, I am just saying what I do. It is another one of those controversial subjects. I feel that once the momma has her babies the most important thing is to make sure the momma eats about 4 good meals a day, the last meal being right before bedtime.
Over supplementing a pregnant female can cause birth defects as can feeding them a poor diet. Here is a interesting article that explains some of those issues:
Birth Defects in Dogs.