Endogenous origin of microbiologically-inactive cobalamins (cobalamin analogues) in the human fetus

The cobalamin content of placenta, maternal and fetal sera was assayed by methods which measure microbiologically-active and microbiologically-inactive cobalamins. Only microbiologically-active cobalamins were present in the placenta but both types were present in both maternal and fetal circulations. We conclude that only microbiologically-active cobalamins cross the placenta and that the microbiologically inactive cobalamins in the fetus must arise by metabolism in situ.