
I don't change the diet at all until they are confirmed in whelp (at about 28 days) then I put them on the puppy food the pups will have, and don't increase at all until the 6th week, and by whelping they are only on half as much as the maintenance amount (but a more nutrient dense food).
It is not good to let a bitch gain extra bodyweight in pregnancy, (can cause whelping problems and over large pups) it is after whelping that she will need lots more food, up to about 4 times her normal ration for an average litter for a medium size breed.