This review examines the importance of the placenta in iron metabolism during development and the effect of iron deficiency on maternal and fetal physiology. Iron is an essential micronutrient, required for a wide variety of biological processes. During pregnancy, the mother has to deplete her iron stores in order to provide the baby with adequate amounts. Trans-placental iron transfer involves binding transferrin (Tf)-bound iron to the Tf receptor, uptake into an endosome, acidification, release of iron through divalent metal transporter 1, efflux across the basolateral membrane through ferroportin and oxidation of Fe(II) by zyklopen. An additional haem transport system has been hypothesised, which may explain why certain gene knockouts are not lethal for the developing fetus. Iron deficiency is a common phenomenon during pregnancy, and the placenta adapts by up-regulating its transfer systems, maintaining iron at the expense of the mother. Despite these adaptations, deficiency cannot be completely prevented, and the offspring suffers both short- and long-term consequences. Some of these, at least, may arise from decreased expression of genes involved in the cell cycle and altered expression of transcription factors, such as c-myc, which in turn can produce, for example, kidneys with reduced numbers of nephrons. The mechanism whereby these changes are induced is not certain, but may simply be as a result of the reduced availability of iron resulting in decreased enzyme activity. Since these changes are so significant, and because some of the changes are irreversible, we believe that iron prophylaxis should be considered in all pregnancies.