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.
Sources:
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"The Effects of Vegetarian and Vegan Diet during Pregnancy on the Health of Mothers and Offspring" (2019) https://pubmed.ncbi.nlm.nih.gov/30845641/
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"Vegetarian or vegan and pregnant" (2022) https://www.nhs.uk/pregnancy/keeping-well/vegetarian-or-vegan-and-pregnant/
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"The Academy of Nutrition and Dietetics Publishes Stance on Vegan and Vegetarian Diets" (2016) https://www.pcrm.org/news/health-nutrition/academy-nutrition-and-dietetics-publishes-stance-vegan-and-vegetarian-diets
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"Plant-based diets and their impact on health, sustainability and the environment" (2021) https://apps.who.int/iris/bitstream/handle/10665/349086/WHO-EURO-2021-4007-43766-61591-eng.pdf