A well-planned vegan diet is safe and beneficial for all stages of life, including pregnancy. That being said, making sure that one is eating a balanced diet (just like a pregnant omnivore would) and paying attention to the intake of vitamin B12 (which is usually found in animal products) are advised. Iron (leafy greens and lots more), vitamin D (mushrooms), calcium (leafy greens and lots more), and iodine (kelp/seaweed/prunes/lima beans) are easier to come by than B12, but should also be payed attention to. All of these can be easily supplemented if necessary.