The placenta is built during pregnancy, performs several jobs at once and is discarded at birth. A large share of antenatal monitoring is directed at it rather than at the baby.
It is an exchange surface
Maternal and fetal blood do not mix. They are brought into close contact across a membrane, and oxygen, nutrients and waste move across that surface.
The structure maximises surface area within a small volume, with fetal vessels branching into spaces bathed in maternal blood. The efficiency of the exchange depends on that architecture.
Anything that reduces blood flow on the maternal side, or damages the branching structure, reduces the supply reaching the baby, which is the underlying issue in restricted growth.
It is also an endocrine organ
The placenta produces hormones that maintain the pregnancy, adjust the mother's metabolism to make more glucose available, and prepare tissues for birth and feeding.
Those metabolic adjustments are why some conditions appear specifically in pregnancy, including changes in how the body handles glucose in the second half.
Hormone production also explains why symptoms follow the placenta's development rather than the baby's, particularly in the first trimester.
The barrier is selective, not complete
Antibodies cross to the baby in late pregnancy, providing early protection, which is why maternal vaccination in pregnancy is used for some infections.
Many other substances cross as well, including some medications, alcohol and components of tobacco smoke. The description of the placenta as a barrier overstates what it filters.
This is the basis for advice about substances during pregnancy, and for the caution around medications, which is why prescribing decisions in pregnancy are made individually.
Position matters as much as function
The placenta implants wherever the embryo settles. If it covers or lies close to the cervix, it affects how a birth can safely proceed.
Position is checked at the mid-pregnancy scan and often rechecked later, because the growing uterus frequently carries a low-lying placenta upward as pregnancy advances.
Separation from the wall of the uterus before birth is a separate and serious event, which is why bleeding in later pregnancy is always assessed urgently.
How it is monitored indirectly
There is no simple direct test of placental performance, so it is inferred from growth measurements, fluid volume, blood flow studies and the baby's movement pattern.
Reduced or changed movements are treated as potentially significant because they can reflect a baby conserving energy, and services ask to be contacted the same day.
After birth the placenta is examined where the pregnancy had complications, since its structure often explains events that were not identifiable beforehand.