By Dr. Jill Hechtman M.D. OB/GYN explains, in a somewhat cheesy and lacking video, that it is completely safe to have a vegetarian diet while pregnant. And since she doesn't mentioned that dairy is necessary, I assume that she believes veganism is OK too.  Her main piece of advice is to make sure you get enough protein and nutrients, recommending beans, vegetables, pre-natal vitamins, and extra supplements. She says, "the [food] pyramid is essentially the same, just take out the meat and add your proteins that will help keep your diet balanced"