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11.1 MONOCHORIONIC/MONOAMNIOTIC TWINNING

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EPIDEMIOLOGY/GENETICS

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Definition Monoamniotic (MA) twins share a common amniotic sac. Triplets and higher-order multifetal gestations also may contain a monochorionic (MC) twin pair. The most severe form of MA twins is conjoined (CJ) twins, which are defined by the shared body region (“pagus”), and can be dorsally (more common) or ventrally conjoined.

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Incidence The reported incidence of MA twinning ranges from 1 in 1650 to 1 in 93,734 live births. Up to 20% of higher-order multifetal gestations can contain a MC pair. CJ twins are the rarest MC complication, occurring at an approximate rate of 10.25 per million births.

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Embryology MA and CJ twins are at the last part of a spectrum of incomplete cleavage. Discordant anomalies occur more commonly in MC gestations (6% to 8%).

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There are several proposed mechanisms for the increased incidence of structural anomalies in MC twins, including teratogenicity of zygotic splitting and vascular “accidents.” Because of their unique angioarchitecture, MC twins have a higher incidence of morbidity and mortality than dichorionic (DC) twins. This is likely due to the sharing of a single placenta and the vascular communications that exist between twins. Loss of one twin can result in death (~15%) or significant neurologic injury (~30%) in the surviving twin. The suspected mechanism is pressure and volume shifts that occur at the time of death of one twin: the surviving twin exsanguinates into the demised twin.

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Inheritance Patterns Inheritance is sporadic.

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Teratogenesis None are known.

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Prognosis The prognosis for MA twins is largely related to the presence of congenital malformations and the gestational age at delivery. Multifetal MC gestations are at a high risk for pregnancy complications and invariably result in preterm birth, either iatrogenic or spontaneous. Only 18% of prenatally diagnosed CJ twins survive.

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SONOGRAPHY

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FINDINGS
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  • MA twins are suspected when an intertwin membrane cannot be identified in association with other sonographic stigmata of an MC gestation.

    • Single placental mass

    • Concordant gender

  • Documenting cord entanglement confirms the diagnosis.

  • Other complications of MC gestations can occur within a MA gestation.

    • Twin-twin transfusion syndrome (TTTS)

    • Selective fetal growth restriction (sFGR)

    • Twin anemia-polycythemia sequence (TAPS)

    • Discordant anomalies

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KEY FINDINGS/PITFALLS
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  • Some major structural anomalies, such as anencephaly and lower urinary tract obstruction, can be reliably detected in the late first trimester.

  • Chorionicity in high-order multifetal gestations is best determined in the first trimester.

  • The diagnosis of CJ twins is relatively straightforward.

    • No intervening amniotic fluid

    • Single umbilical cord with more than three vessels

    • Persistence of heads at the same body plane

    • Lack of separation of fetuses over time

  • Defining the degree of conjoining can be challenging and may require high-resolution two-dimensional ultrasound, three-dimensional ultrasound, color Doppler mapping, magnetic resonance imaging (MRI), and fetal echocardiography.

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DIFFERENTIAL ...

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