Abstract
This is the review of the studies conducted both in humans and rats to elucidate the metabolism of vitamin B_<12> during pregnancy. Vitamin B_<12> is transferred from the mother to the fetus via the placenta which is capable of extracting this vitamin from the maternal blood and sends it into the fetus against a concentration gradient, the fetal blood vitamin B_<12> being much higher than the maternal vitamin B_<12>. The villi of the placenta probably have an active transport system for vitamin B_<12>. It was observed that a cretin was born to a mother who had abnormally low serum vitamin B_<12>. The possible involvement of this vitamin in abnormal fetal development and an interrelationship between vitamin B_<12> and the thyroid are pointed out. The nutritional burden imposed upon pregnant females is also discussed from the nutritional and clinical point of view.
