70% dizygotic : 2 ova fertilized by different sperm; therefore, all are dichorionic

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

Dichorionic 1st trimester: Thick echogenic chorion completely surrounds each embryo

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  2 amniotic sacs, 2 yolk sacs (YS)

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

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

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

Use transvaginal US in 1st trimester to document chorionicity and amnionicity as chorionicity determines prognosis

Look for placenta/vasa previa, marginal/velamentous cord

Monitor growth at least monthly

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

Careful search for anomalies; increased risk in all twins