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  <head>
    <title>085-03 Placenta, fetal membranes, maternal–fetal exchange, twins, and developmental timing</title>
    <ownerName>Integrated Medical Foundations</ownerName>
  </head>
  <body>
    <outline text="Placenta, membranes, twins, and timing">
      <outline text="Placental structure">
        <outline text="Fetal chorion plus maternal decidua"/>
        <outline text="Blood approaches without bulk mixing"/>
        <outline text="Primary, secondary, tertiary villi">
          <outline text="Cytotrophoblast columns, then mesoderm core"/>
          <outline text="Fetal capillaries make tertiary villi"/>
        </outline>
        <outline text="Anchoring villi attach to decidua"/>
        <outline text="Floating terminal villi provide exchange"/>
        <outline text="Barrier layers under syncytiotrophoblast">
          <outline text="Hofbauer macrophages in villous core"/>
        </outline>
      </outline>
      <outline text="Exchange">
        <outline text="Barrier thins as pregnancy advances">
          <outline text="Shorter diffusion distance"/>
        </outline>
        <outline text="Gases diffuse"/>
        <outline text="Glucose by facilitated transport"/>
        <outline text="Amino acids by active or carrier transport"/>
        <outline text="Immunoglobulin G by receptor-mediated transcytosis"/>
        <outline text="Oxygen transfer determinants">
          <outline text="Partial pressure, area, thickness, flow"/>
          <outline text="Fetal haemoglobin, double Bohr effect"/>
        </outline>
        <outline text="Placenta consumes oxygen itself"/>
        <outline text="Not a simple filter">
          <outline text="Waste moves to maternal circulation"/>
        </outline>
      </outline>
      <outline text="Maternal blood supply">
        <outline text="Spiral arteries feed intervillous space"/>
        <outline text="Extravillous trophoblast remodels arteries">
          <outline text="Wide low-resistance channels"/>
        </outline>
        <outline text="Incomplete remodelling">
          <outline text="Growth restriction and pre-eclampsia"/>
        </outline>
        <outline text="Umbilical arteries carry deoxygenated blood"/>
      </outline>
      <outline text="Endocrine and immune roles">
        <outline text="Chorionic gonadotropin maintains corpus luteum"/>
        <outline text="Progesterone suppresses contractility"/>
        <outline text="Oestriol needs fetal adrenal and liver"/>
        <outline text="Placental lactogen shifts maternal metabolism">
          <outline text="Insulin resistance spares fuel for fetus"/>
          <outline text="Excess: gestational diabetes"/>
        </outline>
        <outline text="Corticotropin-releasing hormone times birth"/>
        <outline text="Interface is immunologically active">
          <outline text="Some histocompatibility molecules absent"/>
          <outline text="Uterine natural-killer cells and macrophages"/>
          <outline text="Adjusted, not globally suppressed"/>
        </outline>
      </outline>
      <outline text="Decidua and membranes">
        <outline text="Decidua basalis forms maternal part"/>
        <outline text="Capsularis fuses with parietalis"/>
        <outline text="Chorion frondosum persists, laeve regresses">
          <outline text="Discoid placenta"/>
        </outline>
        <outline text="Cotyledons from decidual septa"/>
        <outline text="Inspect placenta after delivery"/>
        <outline text="Amniotic fluid: urine in, swallowing out">
          <outline text="Oligohydramnios: pulmonary hypoplasia"/>
          <outline text="Polyhydramnios: impaired swallowing, diabetes"/>
        </outline>
        <outline text="Allantois forms urachus">
          <outline text="Remnant: median umbilical ligament"/>
        </outline>
        <outline text="Cord: two arteries, one vein, Wharton jelly"/>
      </outline>
      <outline text="Fetal circulation">
        <outline text="Ductus venosus bypasses liver"/>
        <outline text="Foramen ovale shunts right to left">
          <outline text="Oxygenated blood to heart and brain"/>
        </outline>
        <outline text="Ductus arteriosus bypasses lungs">
          <outline text="High pulmonary resistance"/>
        </outline>
        <outline text="Changes at birth">
          <outline text="Lung expansion lowers pulmonary resistance"/>
          <outline text="Left-atrial pressure closes foramen ovale"/>
          <outline text="Oxygen and low prostaglandins constrict ductus"/>
        </outline>
      </outline>
      <outline text="Twins">
        <outline text="Dizygotic: two oocytes, usually dichorionic"/>
        <outline text="Monozygotic type depends on split timing">
          <outline text="Very early: dichorionic diamniotic"/>
          <outline text="Blastocyst: monochorionic diamniotic"/>
          <outline text="Later: monochorionic monoamniotic"/>
          <outline text="Incomplete late split: conjoined"/>
        </outline>
        <outline text="Chorionicity outweighs zygosity">
          <outline text="Twin-to-twin transfusion"/>
          <outline text="Monoamniotic cord entanglement"/>
        </outline>
      </outline>
      <outline text="Timing and teratogens">
        <outline text="Crown-rump length accurate early"/>
        <outline text="Growth restriction versus small healthy fetus"/>
        <outline text="First two weeks: all-or-none, not absolute"/>
        <outline text="Weeks three to eight: organogenesis"/>
        <outline text="Fetal period: growth, ongoing CNS risk"/>
        <outline text="Outcome depends on dose, timing, genotype"/>
        <outline text="Communicate absolute risk and uncertainty"/>
      </outline>
    </outline>
  </body>
</opml>
