Why. The child would only then not get everything it needs when the mother doesn't get everything she needs. That's a concern that people should have for every diet. Now when you think that you might not get every nutrient, then just make some research and look into what's important. You can find so much information about it. It's not a reason to pay for animals to suffer and die when there is no necessity right?

Break it for the child's sake? What do you mean by that? As long as you keep it safe for the child, there is no issue and therefore no reason to not be vegan during pregnancy

For me, the questions is rather "what supplements do you consider a 'must take' during pregnancy" :)