Placenta and fetal membranes

Placenta and fetal membranes

Placenta and fetal membranes

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Placental Development - The First Hookup

  • Implantation (Day 6-12): Blastocyst adheres to the uterine wall. The trophoblast rapidly proliferates, differentiating into two key layers.
  • Trophoblast Layers:
    • Cytotrophoblast: Inner, mitotically active cellular layer. Serves as a stem cell source for the syncytiotrophoblast.
    • Syncytiotrophoblast: Outer, multinucleated syncytium. Invades the decidua and secretes hCG.
  • Circulation: The syncytiotrophoblast erodes maternal spiral arteries, creating lacunae that fill with maternal blood, establishing uteroplacental circulation.

⭐ The syncytiotrophoblast lacks MHC-I expression, preventing recognition as foreign tissue by the maternal immune system.

Early placental development and uterine wall invasion

Placental Structure & Circulation - The Fetal Lifeline

  • Components: Fetal side (Chorionic plate) and Maternal side (Basal plate, decidua basalis). Intervillous space is between them.
  • Fetal Surface: Smooth, glistening, covered by amnion. Umbilical cord inserts centrally.
  • Maternal Surface: Rough, lobulated (cotyledons).

Placental Villi Structure and Gas Exchange

  • Circulation Pathway:

High-Yield: Maternal and fetal blood do not mix. Gas and nutrient exchange occurs across the placental barrier (syncytiotrophoblast, cytotrophoblast, villus stroma, fetal capillary endothelium).

Placental Functions & Transport - The Border Patrol

  • Core Functions: Gas exchange (O₂, CO₂), nutrition, waste removal (urea), endocrine synthesis, and immune barrier/transfer.
  • Transport Mechanisms:
    • Simple Diffusion: Gases, water, electrolytes, urea, most drugs.
    • Facilitated Diffusion: Glucose (carrier-mediated).
    • Active Transport: Amino acids, vitamins, ions (Fe, Ca, I).
    • Pinocytosis: Maternal IgG antibodies, providing passive immunity.
  • Endocrine Factory: Produces hCG, hPL, progesterone, and estrogens.

⭐ Maternal insulin does not cross the placenta, but glucose does. Uncontrolled maternal diabetes can lead to fetal hyperglycemia and subsequent hyperinsulinemia, causing macrosomia.

Facilitated Diffusion of Glucose Across a Cell Membrane

Fetal Membranes & Amniotic Fluid - The Baby Bubble

Fetal membranes and their cellular layers

  • Amnion & Chorion: Inner (amnion) and outer (chorion) layers enclosing the fetus.
    • Amnion: Secretes amniotic fluid; derived from epiblast.
    • Chorion: Forms fetal contribution to the placenta.
  • Amniotic Fluid Functions: Provides cushioning, permits movement/growth, maintains temperature.
  • Composition & Volume: Initially a maternal plasma ultrafiltrate. From 2nd trimester on, fetal urine is the primary source. Fetal swallowing removes it.

    Oligohydramnios (low fluid, AFI <5 cm) suggests renal anomalies (Potter sequence) or uteroplacental insufficiency. Polyhydramnios (high fluid, AFI >24 cm) suggests impaired swallowing (anencephaly, GI atresia) or high cardiac output.

Placental Abnormalities - When Implantation Wanders

  • Placenta Accreta Spectrum: Abnormal trophoblast invasion into the myometrium due to a defective decidual layer. A major cause of severe postpartum hemorrhage.
    • Accreta: Placental villi attach directly to the myometrium.
    • Increta: Villi invade into the myometrium.
    • Percreta: Villi perforate through the myometrium, potentially invading adjacent organs (e.g., bladder).
  • Key Risk Factors: Prior C-section, placenta previa, advanced maternal age.
  • 📌 Mnemonic: Accreta, Increta, Percreta = Adheres, Invades, Perforates.

⭐ Classic presentation involves massive, life-threatening hemorrhage during attempted manual placental separation. Hysterectomy is often the definitive management.

Placenta Accreta, Increta, and Percreta Invasion Depths

High‑Yield Points - ⚡ Biggest Takeaways

  • The placenta has fetal (chorionic villi) and maternal (decidua basalis) components.
  • Syncytiotrophoblast secretes hCG to maintain the corpus luteum and lacks MHC-I.
  • Maternal and fetal blood do not mix; exchange occurs in the intervillous space.
  • Umbilical cord: two arteries carry deoxygenated blood, one vein carries oxygenated blood.
  • Polyhydramnios links to swallowing defects; oligohydramnios to renal agenesis (Potter sequence).
  • Placenta accreta is the abnormal invasion of trophoblasts into the myometrium.

Practice Questions: Placenta and fetal membranes

Test your understanding with these related questions

A 42-year-old woman presents to the emergency department in active labor. She has had no prenatal care and is unsure of the gestational age. Labor progresses rapidly and spontaneous vaginal delivery of a baby boy occurs 3 hours after presentation. On initial exam, the child is 1.9 kg (4.2 lb) with a small head and jaw. A sac-like structure containing intestine, as can be seen in the picture, protrudes from the abdominal wall. What complication is closely associated with this presentation?

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Flashcards: Placenta and fetal membranes

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Neurulation begins in the _____ week of fetal development

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Neurulation begins in the _____ week of fetal development

third

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