placental disc

The placental disc is the disc-shaped organ that develops during pregnancy to support fetal development. It facilitates the exchange of oxygen, nutrients, and waste products between the maternal and fetal bloodstreams while maintaining structural separation between these two circulations and providing an immunological barrier.

The placental disc is organized so that maternal blood flows through the intervillous space surrounding the chorionic villi, while fetal blood flows within the capillaries inside the villi. Exchange occurs across the villous membrane consisting of syncytiotrophoblast, cytotrophoblast, villous mesenchyme, and fetal capillary endothelium. This hemochorial structure maximizes exchange surface while keeping the two blood supplies completely separate.

The human placenta at term has an enormous total exchange surface area estimated at 10–14 square meters, achieved through extensive branching of the chorionic villi into thousands of terminal villi. This large surface area is critical for supporting the metabolic demands of the fetus, particularly for oxygen transfer, which requires rapid diffusion across a short distance and large area.

The placental disc has two surfaces: the fetal (chorionic) surface, which is smooth, shiny, and covered by the chorionic plate from which the umbilical cord arises; and the maternal (basal) surface, which is rough and divided into 10–40 lobes called cotyledons by fibrous septa, and is in contact with the maternal decidua basalis and spiral arteries.

Placenta previa occurs when the placental disc implants in the lower uterine segment, covering or approaching the internal cervical os. This abnormal position means the placenta lies in front of the presenting fetal part and overlies the birth canal, causing painless antepartum hemorrhage and requiring cesarean delivery to avoid catastrophic bleeding during vaginal birth.