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
I understand that people want to be careful when it comes to their pregnancy and the health of their kid. There's no need to add in any animal products to get the nutrients that a person's body needs, even the increased/slightly different needs during pregnancy.