KEY FACTS
Terminology
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70% dizygotic : 2 ova fertilized by different sperm; therefore, all are dichorionic
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30% monozygotic : Single ovum fertilized by single sperm, which then divides to produce twins
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30% dichorionic, 60-65% monochorionic diamniotic
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5-10% monoamniotic, < 1% conjoined
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Imaging
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Dichorionic 1st trimester: Thick echogenic chorion completely surrounds each embryo
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2 amniotic sacs, 2 yolk sacs (YS)
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Dichorionic 2nd/3rd trimester: Twin peak sign with wedge of chorionic tissue extending from placenta into base of intertwin membrane
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Thick intertwin membrane, 2 separate placentas
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Monochorionic 1st trimester: Single thick chorion surrounds both embryos
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Diamniotic: 2 YS, 2 amnions, 1 around each embryo
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Monoamniotic: 1 YS, 1 amnion surrounds both embryos
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Monochorionic 2nd/3rd trimester must be same sex with single placental mass
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Diamniotic: Thin membrane, T sign of membrane perpendicular to placenta
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Monoamniotic: No membrane, cord entanglement
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Scanning Tips
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Use transvaginal US in 1st trimester to document chorionicity and amnionicity as chorionicity determines prognosis
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Look for placenta/vasa previa, marginal/velamentous cord
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Monitor growth at least monthly
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Check fluid volume every 2 weeks in monochorionic twins to monitor for twin-twin transfusion
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If diamniotic: Maximum vertical pockets on either side of membrane
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Must rely on bladders/Doppler in monoamniotic
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Careful search for anomalies; increased risk in all twins