Calcium is actively transported across the placenta and levels in the fetus are higher than in the mother. Magnesium is also actively transported across the placenta but there does not seem to be a clear relation ship between maternal and fetal concentrations. Zinc is actively transported bound to albumin and is found in significantly higher concentration in the fetal than the maternal circulation. Iron transfer to the fetus is marked, particularly in late pregnancy when the fetal demands are greatest, and there are tranferrin receptors on the placenta which facilitate the transfer of transferrin-bound iron, by endocytosis.