Membranes in Multiple Gestations



Membranes in Multiple Gestations


Anne Kennedy, MD



DIFFERENTIAL DIAGNOSIS


Common



  • Dichorionic Diamniotic Twins


  • Monochorionic Diamniotic Twins


  • Monochorionic Monoamniotic Twins


Less Common



  • Bleed Between Membranes


  • Synechiae


  • Chorioamniotic Separation


  • Amniotic Band Syndrome


Rare but Important



  • Uterine Duplication


ESSENTIAL INFORMATION


Key Differential Diagnosis Issues



  • Twin types



    • 70% dizygotic: Two separate fertilized ova


    • 30% monozygotic: Single zygote splits at various times post conception



      • Zygote splits at < 3 days post conception → dichorionic twins (30%)


      • Split occurs 4-8 days post conception → monochorionic diamniotic twins (60-65%)


      • Split occurs 8-12 days post conception → monoamniotic twins (5-10%)


      • Split occurs > 13 days post conception → conjoined twins (< 1%)


  • Dichorionic twins have best prognosis with high likelihood of two liveborn infants


  • Important to recognize monochorionic twins due to specific complications



    • Twin twin transfusion syndrome (TTTS)


    • Twin reversed arterial perfusion (TRAP) sequence


  • Important to recognize monoamniotic twins as high risk of cord accidents, as well as other complications of monochorionic twinning


Helpful Clues for Common Diagnoses



  • Dichorionic Diamniotic Twins



    • First trimester: Two complete thick echogenic chorionic rings


    • Thick membrane



      • 2 layers chorion + 2 layers amnion = 4 layers = thick membrane


      • In second and third trimester use high frequency transducer to count layers if chorionicity otherwise unclear


    • “Twin peak” or “lambda” sign



      • Echogenic chorionic tissue extends between layers of amnion


      • Triangle shape with base on placental surface


      • Apex of triangle fades into inter-twin membrane


    • Two placentas



      • Can be difficult to determine if placentas implanted side by side or fused


    • Different genders = dizygotic = dichorionic by definition


  • Monochorionic Diamniotic Twins



    • First trimester: One thick echogenic outer ring containing two thin rings of amnion



      • Count yolk sacs if amnion not yet visible


      • Number of yolk sacs = number of amnions


    • Thin membrane



      • 2 layers of amnion + no chorion = thin membrane


    • “T” sign



      • Thin membrane abuts placental surface at 90° without echogenic triangle of chorionic tissue


    • Single placenta



      • Beware pitfall of fused placentas in dichorionic twins


      • Do not confuse succenturiate lobe with second complete placenta


    • Monochorionic twins must be same gender


  • Monochorionic Monoamniotic Twins



    • First trimester: One thick echogenic ring containing two embryos inside a single amniotic sac with single yolk sac


    • No inter-twin membrane



      • Beware pitfall of TTTS with anhydramnios of one twin


      • “Stuck” twin stays adherent to uterine wall


      • Membrane difficult to see: Look carefully between extremities or where head is adjacent to uterine wall


    • Monoamniotic twins free to move around each other



      • Cord entanglement only occurs in absence of membrane



    • Single placenta



      • Cord insertion sites often closely approximated


Helpful Clues for Less Common Diagnoses



  • Bleed Between Membranes



    • Apparent thick membrane but echogenicity < chorion


    • Echogenicity and thickness changes quite rapidly as clot breaks down


  • Synechiae



    • Ask patient about prior surgeries


    • In first trimester, can cause distorted sac shape


    • Synechiae cross uterine cavity, may look like thick membrane



      • In singleton pregnancy, look for fetus straddling synechia or placenta on one side/fetus on the other


      • In twin pregnancy look at relationship to placenta: Membranes reflect off placenta, synechial attachment to myometrium can be anywhere


  • Chorioamniotic Separation



    • Visualization of chorion and amnion as separate layers after 14-16 weeks


    • May be associated with aneuploidy when seen early in second trimester


    • More usually complication of intervention



      • Amniocentesis


      • Amnioreduction in TTTS or for polyhydramnios in one twin


      • Hysteroscopic laser ablation of abnormal vascular connections in TTTS or TRAP


    • Chorioamniotic separation in singleton pregnancy



      • One fetus with “membrane” tending to parallel margins of cavity rather than divide it


      • Do not confuse with failed twin gestation


  • Amniotic Band Syndrome



    • Very variable presentation: Constriction bands on extremities to major amputation/destructive defects


    • Look for fine filaments in amniotic fluid


    • Fetus often “tethered” by bands; may need to vary maternal position to float fetus away from uterine wall


Helpful Clues for Rare Diagnoses

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Aug 10, 2016 | Posted by in OBSTETRICS | Comments Off on Membranes in Multiple Gestations

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