Placenta

The placenta develops from both fetal and maternal tissue. Its fetal portion arises from the chorion, whose chorionic villi branch into the maternal decidua basalis and bathe in maternal blood within the intervillous space. Maternal and fetal circulations run adjacent to one another but do not normally mix; exchange occurs across the thin villous membrane by diffusion, facilitated transport, and active transport.

Four functions define the organ. It performs respiratory exchange, moving oxygen to the fetus and carbon dioxide back to the mother. It performs nutritive and excretory work, transferring glucose, amino acids, and other nutrients while clearing urea and other wastes. It acts as a partial barrier, blocking many pathogens and large molecules while allowing maternal IgG through to confer passive immunity — though alcohol, many drugs, and some organisms such as rubella and Toxoplasma still cross. Finally, it is an endocrine organ, secreting hCG, human placental lactogen, progesterone, and estrogen; hCG maintains the corpus luteum early on until the placenta itself takes over progesterone production.

The umbilical cord links fetus to placenta and normally contains three vessels: two umbilical arteries carrying deoxygenated blood from the fetus to the placenta, and one umbilical vein carrying oxygenated blood back to the fetus. Note that the naming follows direction of flow relative to the heart rather than oxygen content. After delivery of the newborn, the placenta separates and is expelled during the third stage of labor, and it is inspected to confirm it is intact, since retained fragments cause postpartum hemorrhage. Placental abnormalities such as placenta previa and placental abruption are important causes of antepartum bleeding.

USMLE Step 1 covers placental development in embryology alongside the umbilical cord and fetal circulation. NCLEX addresses the placenta throughout antepartum and intrapartum care, including third-stage management and placental complications, while the CCMA exam touches on it in obstetrics and gynecology content.