If you have ever bred and raised animals like I have then you would know that proper nutrition is absolutely essential for the success and health of both the mother and the offspring. With the proper diet you don't have to add any prenatal vitamins to their diet.
Most people find out from their vets that you feed puppy food to pregnant dogs and kitten food to pregnant cats. The reason for that is that the "growth" formula foods have all the nutrition that the mother needs to pass on to her unborn offspring. Don't take my word for it – ask your vet.