Giving vitamin supplements without veterinary oversight comes with potential dangers. Any premium food formulated for growth is also balanced for pregnant and lactating bitches. The increase of calcium is needed to maintain homeostasis within the pregnant dog and provide enough calcium for her developing puppies.
If your new mom is eating a balanced diet she should be getting all the nutrition she needs for herself and for her babes while shes nursing. In general your dog will not need any special vitamins or supplements while she is pregnant as long as she is getting the proper diet. For example folic acid and iron.
Doc Roys Daily Care is good if you do not want to switch up your routine but the Oxy products are better during this time for the females. If you are feeding her a high quality food such as Royal Canin Hlls Science diet or Eukanuba my top. The pups should be removed from the mother for 24 hours and the mother should start calcium and possibly vitamin D supplementation.
Although calcium is very important for the bones and marrow when it comes to dog pregnancy it is highly advisable to skip the calcium supplements except the Vet advises otherwise. But before you go in search of recipes for pregnant dogs or reach for dog prenatal vitamins know this.