Feeding your cat puppy chow will likely result in nutrient deficienies. There are certain essential nutrients in cat foods that are not added in dog foods. I would guess that feeding puppy food to a pregnant kitty could result in still born kittens or very sick kittens at the very least.
Feeding puppy chow will likely cause severe birth defects in the kittens due to taurine deficiency. There are many essential feline nutrients that are absent or deficient in dog/puppy food, including taurine, arginine, Vitamin A, protein, and others. A pregnant cat needs a high quality food for intended for either cats of "all life stages" or for kittens.