An appreciation of the physiologic mechanisms that play a major role in the transfer of substances across the placenta is basic to understanding the capacity of the placenta to respond to the stress of maternal malnutrition during pregnancy and what the nature of this response may mean for the developing fetus.
In general, the homeostatic responses of the mother are geared primarily to her survival. For example, many maternal physiologic adjustments are not only in a direction that will not prevent an untoward change for the fetus, but may even result in a change clearly unfavorable for the fetus. This is exemplified by vasoconstriction of the uterine circulation by catecholamines released into the maternal blood.
Alternatively, the biological capacity of the fetus to adjust to changes in the maternal environment and to maintain an optimal fetal environment is to a considerable degree dependent on and limited by placental mechanisms for