Obstetric Topic 55. The placenta

I. Definition and Structure

Definition

• Placenta: temporary fetomaternal organ for respiration, nutrition, excretion, endocrine regulation and immune interaction.
• Placental circulation starts early, around day 17 after fertilization, and matures through pregnancy.
• Maternal and fetal blood normally remain in separate circulations → exchange occurs across the placental barrier.

Components

• Fetal part: chorion frondosum → chorionic plate, villi and fetal vessels.
• Maternal part: decidua basalis → basal plate.
• Chorionic villi:
  • Cytotrophoblast: inner cellular trophoblast layer.
  • Syncytiotrophoblast: outer multinucleated layer → invasion, maternal-blood contact and hormone secretion.
  • Terminal villi: main exchange units.
• Intervillous space: maternal blood; fetal blood flows through capillaries within villi.
• Term placenta: discoid, with multiple maternal cotyledons.

II. Placental Circulation and Exchange

Two Separate Circulations

1. Uteroplacental circulation: maternal spiral arteries → intervillous space → endometrial veins.
2. Fetoplacental circulation: fetal vessels in chorionic villi → umbilical vein → fetus; umbilical arteries return deoxygenated blood to the placenta.

• Mother → fetus: O2, water, electrolytes, glucose, amino acids, lipids, vitamins and hormones.
• Fetus → mother: CO2, urea, creatinine, bilirubin and other waste products.
• Transfer mechanisms: passive diffusion, facilitated diffusion, active transport and endocytosis.
• IgG crosses the placenta → passive fetal immunity; many drugs, alcohol, pathogens and toxins may also cross.

III. Placental Functions

Endocrine Function

• hCG: produced by syncytiotrophoblast → maintains corpus luteum and progesterone secretion in early pregnancy.
• Progesterone: corpus luteum first, then placenta from about 7-10 weeks → maintains decidua and uterine quiescence.
• Estrogens: uterine/breast growth and uteroplacental blood flow.
• hPL (human placental lactogen): promotes mammary development and maternal insulin resistance/lipolysis → increases glucose availability to the fetus.

Other Functions

• Respiratory: O2 delivery to fetus and CO2 removal.
• Nutritional: transport of glucose, amino acids, fatty acids, vitamins and minerals.
• Excretory: fetal waste transfer to mother.
• Immunologic/metabolic: selective barrier, IgG transfer, storage/synthesis and metabolism of hormones, drugs and toxins.

IV. Exam-Focused Clinical Points

• Placental insufficiency → impaired exchange → fetal growth restriction, oligohydramnios, fetal hypoxia and abnormal umbilical artery Doppler.
• Maternal blood does not directly mix with fetal blood, but fetomaternal hemorrhage may occur → Rh sensitization and hemolytic disease.
• Exam sequence: decidua basalis + chorion frondosum → villi with fetal capillaries → intervillous maternal blood → exchange across the placental barrier.