Placental development begins right after fertilization and implantation. The outer layer of the blastocyst, known as the trophoblast, rapidly differentiates into two distinct layers as it burrows into the uterine decidua: the inner syncytiotrophoblast, a multinucleated protoplasmic mass without distinct cell boundaries, and the outer cytotrophoblast, a layer of active, mononucleated cuboidal cells.
As the syncytiotrophoblast erodes maternal capillaries, spaces called lacunae form within it. These spaces soon merge with maternal sinusoidal blood vessels, establishing the very first primitive uteroplacental circulation around the end of the second week. Mesenchymal tissue then grows into the primary villi, transforming them into secondary villi, and fetal blood vessels begin to differentiate within this mesenchyme, creating tertiary villi by the end of the fourth week.
At term, a normal human placenta is a discoid organ weighing roughly 500 grams, measuring about 15 to 20 centimeters across and roughly 2 to 3 centimeters thick at its center. Examination of a delivered placenta reveals two distinctly contrasting surfaces:
The functional units of the placenta are the chorionic villi. Structurally, these branching projections evolve continuously throughout gestation to maximize surface area for exchange.
As pregnancy progresses toward term, the cellular layers covering these villi thin out considerably. The cytotrophoblast cells become sparse, and the syncytiotrophoblast forms thin epithelial plates directly adjacent to fetal capillary endothelium, shortening the diffusion distance.
The placental membrane (often called the placental barrier) separates maternal blood in the intervillous space from fetal blood inside the capillary lumen. In early pregnancy, this membrane consists of four distinct layers:
By the third trimester, the cytotrophoblast cells largely disappear in many areas, and the membrane thins down to just the syncytiotrophoblast, a minimal basement membrane, and the capillary endothelium. Despite its name as a "barrier," most substances—including drugs, alcohol, and certain infectious agents—can cross it, meaning it is selectively permeable rather than an absolute block.
The maternal side of the placenta is divided into anatomical units called cotyledons. Each cotyledon receives maternal blood through several spiral arteries that pierce the decidual plate, pumping blood directly into the intervillous space under maternal arterial pressure.
Once in the intervillous space, maternal blood washes over the branching terminal villi, allowing gas and nutrient exchange before draining out through endometrial venules. The partitioning of the placenta into multiple semi-independent cotyledons is clinically significant: if a localized area of the placenta experiences infarction or damage, neighboring cotyledons can often maintain adequate fetal oxygenation and support normal pregnancy continuation.