Vasa previa (VP): Umbilical (fetal) vessels under membranes < 2 cm from internal os

90% associated with low-lying placenta

Can occur in 2 different scenarios

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  Velamentous cord insertion

  
  
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  Vessels traveling over/near cervix between main placenta and accessory (succenturiate) lobes

Transvaginal US + color Doppler are best tools

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  Pulse Doppler necessary to prove visualized vessels are fetal and not maternal in origin

Low-lying placenta or placenta previa without VP

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  Maternal vessels near or cover os (not fetal)

  
  
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  Marginal sinus vessels of placenta often confused for fetal vessels

Up to 50% mortality if VP is not diagnosed prenatally

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  Fetal hemorrhage because unprotected umbilical vessels prone to compression and tear

Alternatively, 97-100% survival if VP is diagnosed prenatally

Look for VP with color and pulse Doppler if low-lying placenta diagnosed

Look for low succenturiate lobes

Identify placental cord insertion in all cases

Obtain pulsed Doppler when suspecting VP to prove it is same as fetal heart rate

Beware of mimickers

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  Placental marginal vessels (along edge of placenta, not submembranous)

  
  
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  Uterine arterial or venous flow (deeper than VP vessels)

  
  
  
  
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    Arterial flow matches maternal heart rate; venous flow changes with maternal Valsalva