Actually, a vegetable based diet can be entirely natural (although I’m not quite sure what would be “unnatural” about it) as humans are not carnivores but omnivores.
http://www.vrg.org/nutshell/omni.htm
As an ovo-lacto vegetarian, vitamin b12 deficiency is not of such a concern for me. But, I am surprised that women following a vegan diet would not be aware of the need for a B12 replacement source - regardless if they were pregnant or not. And B12 supplements can be found in items that are not meat - such as nutrional yeast, fortified soy milk and cereals.
http://www.vrg.org/nutrition/b12.htm
I’m not sure what you mean about calculations as to what goes into each meal. Are you talking about the protein complementary theory that talked about complete vs. incomplete proteins? That idea has been ruled out as total bunk. Vegetarians need to follow a food pyramid not all that different from those advocated for meat eaters.
I’m 8 months pregnant and I’ve not consumed any meat during this pregnancy. I was vegetarian before I got pregnant and have continued to be one. I do consume dairy and eggs. But, some of my friends were concerned about my iron levels as anemia is a concern during pregnancy. Many of them asked if I was going to start eating meat again. But, I started out with higher than average levels of iron for a woman my age (most women my age are anemic) and have been able to continue my high iron levels without the need for iron supplements. Most people assume you can only get iron from meat but actually legumes are a much better source.
I hate seeing articles like this because most people interpret it with their own biases - that vegans are crackpots and not healthy. But, for every study like this, I remind myself that 60% of the US is overweight and obesity among children has doubled in the past 20 years. Veganism doesn’t seem to be the rampant problem here. Children stuffing themselves with nutrionally bankrupt fast food and soda does.