The concentrations of vitamin B12 in serum obtained from the umbilical cord blood in 25 fetuses were compared with concentrations found in corresponding samples of maternal serum at the time of delivery. In every case the concentration in the fetal serum was higher than that in the maternal serum. This suggested an active transfer or drawing of vitamin B12 from the mother into the fetus. In 14 pregnant women, most of them in active labor, the intramuscular injection of crystalline vitamin B12 resulted only in a temporary elevation of the vitamin B12 concentration in the maternal blood above that in the fetal blood; after six hours the fetal levels were again higher than the maternal. Other data reviewed here indicate likewise that the fetus draws certain nutriments from the maternal source; this applies to certain vitamins and iron but not to glucose or inulin.
Chow BF, Okuda K. TRANSFER OF VITAMINS FROM MOTHER TO FETUS. JAMA. 1960;172(5):422-426. doi:10.1001/jama.1960.03020050014005