Pathology of the placenta.

The placenta has a considerable functional reserve capacity, easily repairs ischaemic damage, is able to compensate for toxic injury and does not appear to age. Most of the macroscopically visible abnormalities of the placenta are of no functional significance, the major exception to this general banality being the uncommon large haemangioma which can cause complications in the mother, fetus and neonate. Most of the histological abnormalities seen in the placental villi represent a reaction to alterations in either maternal or fetal blood flow through the placenta, but a failure of adequate maturation of the villous tree may impair the functional efficiency of the placenta, as may defective trophoblastic differentiation. Infections of the placenta are important but do not influence placental function, whilst there is currently no firm evidence that the placenta ever suffers immune-mediated damage. Intrinsic placental 'insufficiency' is extremely rare and it is becoming increasingly clear that this clinical syndrome is usually due to a restricted supply of maternal oxygen and nutrients as a result of inadequate transformation of the spiral arteries into uteroplacental vessels. This failure of placentation represents an abnormality of the relationship between fetal and maternal tissues at a relatively early stage of pregnancy, and it is only by gaining a better understanding of this relationship that the problems posed by such conditions as pre-eclampsia and idiopathic intrauterine growth retardation will be answered.