Vegans are more likely to be deficient in this vitamin as it is found mainly in animal products.
Vitamin B12 is necessary for the body's vital functions including the production of red blood cells and cellular metabolic energy.
The human body is not capable of producing vitamin B12 itself, instead we need to get it from our diets. It is found in meat, milk and eggs.
Doctor Tormod Rogne, lead author on the study, said: "Pregnant women who consume too few animal-derived foods increase their risk of developing a vitamin B12 deficiency.".
Although he stressed that there may be other factors at play in causing the deficiency.
It is important to note that this news does not affect vegetarians, only vegans, as another of the researchers Professor Vibeke Videm stated: "B12 deficiency is not common in vegetarians who consume dairy products or eggs, because they can easily meet the recommended B12 intakes through these foods."
The comprehensive study looked at 11,216 pregnancies across 11 countries, and found that globally, low birth weight and preterm births cause 50% of all infant deaths in the first 28 days after birth.
It is not known whether taking B12 supplements have an effect on pregnancy outcome as previous studies have worked with small sample groups.
NHS Choices advises vegan mothers to make sure they get enough B12 from yeast extracts (like Marmite) fortified breakfast cereals and fortified unsweetened soya drinks.