During the pregnancy, the development of the embryo/foetus requires energy, oxygen and nutrients from the mother. It also needs to get rid of waste such as CO2.
This placenta is a special ‘organ’ located in the pregnant uterus and provides an ‘intimate’ correspondence between the maternal and the foetal blood.
From the umbilical veins (from the foetus), blood vessels protrude into numerous villi (plural for villus) where they end as capillaries before the blood streams back to the placental arteries.
From the other, maternal side, blood vessels come very close to the villi and the capillary fluid exits the maternal capillaries and streams around the placental villi.
This diffusion of extravascular fluid provides the opportunity for oxygen, carbon dioxide, nutrients and waste products to mix and stream towards the region with their lowest values, either towards the placenta or towards the maternal circulation.
However, although the capillaries of mother and child come as close as possible towards each other, it is crucial that they must NOT exchange blood!
Why not? Because the embryo/foetus is an individual organism with its own DNA. After all, the foetus shares its DNA from its mother and its father.
For example, every individual has a specific blood type; A, B, AB, or O (see https://www.basicphysiology.org/d-6-blood-types)
It is quite possible that the foetus blood type is different from that of the mother.
If the blood of the foetus would mix with that of the mother, then this difference in blood type would cause, immediately, coagulation. This would certainly cause the death of the foetus and maybe that of the mother too!
This is just one example why it is crucial that the foetus and the mother blood must not mix with each other.
The placenta does not only act as a ‘communicator’ between the mother and the foetus. It also produces several hormones during pregnancy, such as progesterone, oestrogen, gonadotropin and a few more.
All these good stuff (oxygen, nutrients, hormones, etc) flow from the placenta, through the umbilical vein (!) to the foetus.
Ah! Another surprise! In the general circulation, we are used to the fact that arterial blood pumps oxygen and nutrient rich blood at high pressures to the tissues while venous blood streams carbon dioxide and waste products at lower pressures back to the heart.
The names ‘arteries’ and ‘veins, are determined by the pressure of the blood, NOT by its contents.
In the placenta circulation, the two umbilical arteries have a higher pressure than the pressure in the umbilical veins. Remember!
But, as usual in medicine, something can go wrong, also with this organ.
If it attaches itself opposite to the cervix then no problem. During the delivery, there is no obstruction for the foetus to be delivered.
But what if the placenta is positioned near or on the cervix? Then several bleeding can occur in the later pregnancy or during delivery. This situation is called placenta previa. (previa or praevia = before ‘the opening’)
Solution? Caesarean section.
Another problem? Placenta accreta. In this case, the placenta is too firmly attached to the uterus wall and (parts of it) remain after delivery.
Finally, placental insufficiency can be another problem for the foetus. There can be multiple reasons for a poor performance of the placenta such as alcohol abuse, smoking, high blood pressure, etc.
