During pregnancy, you want your bitch's food to provide the correct amount of calcium and correct ratio of calcium to phosphorous. Too much calcium and your bitch may experience eclampsia after whelping. Too little calcium and the pups won't grow properly.

Ideal Amount: 3 grams calcium for every 1000 calories of dog food

Ideal Ratio: 1:1 to 1.2:1 although you can go as high as 1.3:1 if you have no other options. 

To find the ratio, you need to divide the % of calcium by the % of phosphorous. This calculator will do it for you.