Placenta previa (PP) : Placenta covers cervix internal os (IO)

Low-lying placenta (LLP) : Placenta edge < 2 cm from IO

Normal: Placenta edge 2 cm or more from IO

Avoid “marginal” or “partial” PP (deemed confusing)

Placental tissue near cervix is hallmark finding

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  Sagittal view is best

Transvaginal US (TVUS) is essential for diagnosis

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  Best way to evaluate lower uterine segment (LUS)

  
  
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  Measure distance between placental edge and IO

  
  
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  Marginal placental vessels count as placenta

Important associations

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  Morbidly adherent placenta (placenta accreta spectrum)

  
  
  
  
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    ↑ risk if prior cesarean section (C-section)

    
    
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    Greater risk if multiple prior C-sections

  
  
  
  
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  Vasa previa: Fetal vessels cross IO

Overdistended maternal bladder: Pushes front and back of uterus together; mimics long cervix and low placenta

Focal myometrial contraction: Shortens uterus, “pulls” placenta low

Placental abruption: Isoechoic blood clot mimics placenta

LLP or PP is seen in 2% of all midgestation studies

> 90% will resolve: Follow-up at 32 and maybe 36 weeks

C-section for PP, LLPs might deliver vaginally

Scan whole uterus before determining placenta location

TVUS if any placenta seen in LUS

Use color Doppler: Find cord insertion, rule out vasa previa

Move baby head out of pelvis

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  Put bed in Trendelenburg (head lower than feet)

  
  
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  2nd person in room to push baby head up