Researching B12 I found some fascinating info...
According to a 1988 paper in the America Journal of Clinical Nutrition "There is no active vitamin B-12 in anything that grows out of the ground; storage vitamin B12 is found only in animal products where it is ubiquitous and where it is ultimately derived from bacteria." Also, according to the paper, it was found that Iranian vegans who were strict vegetarians (vegans) for religious reasons did not get vitamin B-12 deficiency. The scientist "went to Iran and found that they grew vegetables in night soil (human manure)" and "the vegetables were not carefully washed so they retained B-12 from the manure rich soil which was adequate to prevent B-12 deficiency."
Doing some research, it looks like there are some nutritional sources of B12 that vegans can use. Yeast, while not an animal, is alive and can be grown to be rich in B12 and incorporated in the diets of vegans. Red Star T-6635 is a nutritional yeast that is grown on a B12 enriched medium and has a cheesy taste and texture of parmesan cheese. Further research suggests it is supplemented with B12 and it is risky to rely on these for supplementation for many reasons. But, there are many sources which advocate that all vegetarians monitor their B12 status and consider supplementation. You can read more on B12 Analogues and Deficiency which suggests all vegetarians get their B12, methylmalonic acid and homocysteine checked to be sure.
But, it's not only vegans and vegetarians who are at risk for low B12 levels. Because ourGFfamily can't eat gluten, meat is a staple in our house--which anyone who knows me well is probably aware of. Despite the fact we all eat meat, 3 out of 4 in our family have been low in B12, which is why I am so interested in this vitamin. It turns out, we just don't absorb the vitamin well and need supplementation. Many people with gut issues and digestive disorders like celiac do not absorb B12 well. Could it Be B12: An Epidemic of Misdiagnosis is a fascinating book with an overview of the many conditions associated with low B12.
Interestingly, according to the paper in the American Journal of Clinical Nutrition, because of reabsorption it takes about 20 years to run out of B12 if you stop consuming B-12 but "only 3 years to run out and get vitamin B-12 deficiency if one stops absorbing the vitamin." So people who have digestive problems are at the most risk.
Earthsave Canada has this recommendation for pregnant women and children:
"Pregnant and nursing women need to make sure that both they and their newborns get a reliable source of B12 in their diets every day. Babies need it every day because they don't yet have the stores of the vitamin to draw on like adults do. B12 needs to be in the mother's diet every day during pregnancy and lactation to allow for the child's growth and development."