Placental physiology: Ontogenic changes in the nutritive function throughout pregnancy

In addition to placental growth during the first half of pregnancy, remodelling of the placenta which takes place in the second half of pregnancy contributes significantly to the increase in transport capacity in late pregnancy. Due to the exponential growth of the fetus the balance between supply and demand is shifted towards increasing demand and fetal blood concentrations of important substrates like glucose and oxygen decline. The decrease in fetal blood levels of glucose and oxygen resulting in an increased transplacental gradient add to a rise in flux from the mother to the fetus. The change in distribution of the total uptake from maternal blood between placenta and fetus allows a further increase in the amount of substrate which is available to the fetus and helps to balance the equilibrium between supply and demand. Disturbances in growth and development of the placenta in early pregnancy lead to an earlier and more pronounced manifestation of the compensatory mechanisms supporting supply of the fetus, which normally are only seen in late pregnancy. An increase in the ratio of fetal to placental weight is an indicator of the compensatory mechanisms.