First, let us reassure you of the safety. The Academy of Nutrition and Dietetics (formerly known as the American Dietetic Association) says it clearly: “Well-designed vegetarian diets, that may include fortified foods or supplements, meet current nutrient recommendations and are appropriate for all stages of the life cycle, including pregnancy, lactation, infancy, childhood, and adolescence.” This statement confirms a large amount of experience from vegetarians and vegans the world over. In fact, vegetarian, especially vegan, mothers have more high-birth-weight and fewer low-birth-weight babies than non vegetarian mothers.
Regarding the health advantages, vegan mothers have a risk of preeclampsia 300 times lower than mothers following a diet with foods derived from animals. Vegetarians in general have only half the risk of excessive gestational weight gain. But let’s remember that moms with 9 months of pregnancy and perhaps several more of lactation also have longer term health needs. By following a vegetarian diet, she will be reducing her risk of a number of ordinary diseases such as heart disease, diabetes and some forms of cancer.
The most important supplement for veg moms is vitamin B12. However, a good general prenatal vitamin source is a nutritional insurance policy for those who have a busy lifestyle and can’t always eat the food they ideally need. Although pregnant women have better conversion rates of omega 3 fatty acids to DHA than others, it’s still important to have a good supply of omega 3’s in the diet. Fish are not the best source of Omega 3’s, due to the mercury and other pollutants, not to mention the saturated fat and cholesterol they contain. Good sources include flax and chia seeds, along with walnuts, soy products and canola oil.
See professional level information on veg diets and pregnancy