Doctors, nutritionists and other health nuts all sing the praises of a vegan diet for everyone – with one notable exception of the population: pregnant women.

The problem with a vegan diet, of course, is the risk of vitamin B12 deficiency. B12 can only be obtained naturally from animal products. Children and adults can maneuver around this ably with supplements, but a B12 deficiency during pregnancy can have dire consequences for the fetus. A recent study of 11,216 pregnancies from 11 countries concluded that low levels of vitamin B12 are associated with an increased risk of preterm birth.

Low levels of vitamin B12 in pregnant women do not affect the birth weight of a newborn, but it is associated with a 21 percent increased risk of giving birth prematurely. This was determined in a study by ,” said Tormod Rogne, a medical doctor and intern at Akershus University Hospital near Oslo.

Unlike vegetarians who will eat eggs and dairy products, vegans eat no foods of animal origin whatsoever and are thus susceptible to B12 deficiency if they do not take supplements – regardless of age, gender and whether or not they are pregnant.

Vitamin B12's contribution to your biochemical system includes key roles in the production of red blood cells and cellular metabolic energy. Absent sufficient amounts of B12, you will become anaemic and suffer severe damage to your nervous system. Your body is unable to produce this vital nutrient on its own.

But can't pregnant women who supplement with B12 dodge around these nutrition issues for their unborn children just as they do for themselves? The answer is “maybe.”

“Before we can say anything about the effect of vitamin B12 supplementation in pregnancy, more of these kinds of studies need to be done, and the results should then be summarized in a review article,” wrote Rogne. He notes that in Western countries such as his own Norway, only a small percentage of pregnant women have a vitamin B12 deficiency, whereas in countries such as India which have a pervasive vegetarian culture, the percentage of pregnant women with B12 deficiency can exceed two thirds.

The study was published in the American Journal of Epidemiology.