What is the Placenta?

By Dr. Ananya Mandal, MDReviewed by April Cashin-Garbutt, MA (Editor)

The placenta is a disc shaped organ that has a very rich blood supply that provides nutrition to the unborn baby within the mother’s womb. It weighs around 600 grams when fully formed and is around 2 to 4 cm thick.

Parts of the placenta

There are two sides and two parts of the placenta:-

• The maternal or mother’s side

This forms less than a fifth of the placenta in total. There are compressed sheets on this side called decidua basalis. On these there are grooves much like the surface of a cauliflower that divides it into cotyledons.

• The fetal or baby’s side

There are finger like processes called villi. These project into pools of mother’s blood that are present in the intervillous spaces. The baby’s side is covered with the amniotic membrane.

Types of placenta

• Extrachorial type of placenta

In this type of placenta the chorion does not extend till the edges of the placenta. These may be Circumvallate where a small central chorionic ring appears to be surrounded by thickened amnion and chorion. This may lead to early separation from the walls of the uterus and lead to bleeding.

Another type is Circummarginate where there are central attachments of the amniotic membranes but no central ring.

Accessory types and abnormal placenta

• Succenturiate where an accessory cotyledon forms separately and is attached to the main placenta.
• The placenta may be Bipartite or divided into two lobes but connected with blood vessels.
• The placenta may be Annular or ring shaped.
• Placenta previa or abnormal implantation of the placenta into the lower uterine segment near the cervix. This type of placenta may present an obstruction to the delivery of the baby leading to heavy bleeding if it completely covers the cervix opening.

• Abruptio placentae or early separation of the placenta from the uterus walls leading to heavy bleeding and death of the fetus and/or the mother unless immediate steps are taken to correct the bleeding.
• Placenta accreta or abnormal attachment of the placenta. The villi of the placenta are attached to the uterine muscles instead of in the maternal pools of blood in the intervillous spaces. This may lead to non separation of the placenta at birth, maternal bleeding, fetal death and other complications.

Further Reading

• All Placenta Content
• Placental Microbiome
• Placenta Accreta
• Placental Expulsion - Active vs. Physiological Management