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4.2.2 Antenatal Identification of Placental Location

Sonography

  • Ideally sonography is used for antepartum identification of placental location​
  • Timing: Diagnosis of placenta previa or low-lying placenta should NOT be made <18 to 20 weeks GA, and provisional dx should be confirmed >32 weeks GA, or sooner if clinically warranted (SOGC Guideline 402).​
  • Method: Assessment by transvaginal ultrasound is recommended in all cases where placenta previa or a low-lying placenta is present or suspected by transabdominal sonography, with attempt to clearly define placental location (including laterality), characteristics of placental edge (including thickness, presence of a marginal sinus), and associated findings (succenturiate lobe, cord insertion close to the cervix) (SOGC Guideline 402).​
  • Further Imaging: Pregnant women with clinical risk factors for placenta accreta spectrum disorders and anterior placenta previa at the 18–20-week fetal anatomical ultrasound should be referred for specialist imaging to diagnose or exclude this disorder (SOGC Guideline 383).​

Painless antepartum bleeding

  • Antepartum bleeding, typically painless bleeding, should prompt investigation of possible abnormal placentation.​
  • No digital examinations antepartum until placental location is confirmed!
  • Visualization of placenta through the cervical os on speculum exam warrants immediate action.​

Heavy Bleeding in Labour

Williams describes attempting a vaginal exam in the OR with an emergency CS setup prepared to palpate the placenta (if NO ultrasound available)​

Updated on June 28, 2021

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