In placenta previa, is the preferred ultrasound (US) approach abdominal or vaginal?

Ultrasound Approach for Placenta Previa

In placenta previa, the initial evaluation should be performed with transabdominal ultrasound, followed by transvaginal ultrasound which is the preferred method for definitive diagnosis and detailed evaluation, especially when transabdominal images are inadequate or inconclusive. 1, 2

Diagnostic Approach Algorithm

Step 1: Initial Assessment

• Begin with transabdominal ultrasound for initial screening and placental localization
• Assess placental position relative to the internal cervical os
• Evaluate for any vessels overlying the internal os

Step 2: Follow with Transvaginal Ultrasound

• Transvaginal ultrasound provides superior visualization and diagnostic accuracy
• Particularly valuable for:
  • Posterior placentas (which are often shadowed by fetal parts on abdominal ultrasound) 3
  • Precise measurement of distance between placental edge and internal os
  • Detailed evaluation of the lower uterine segment 1

Step 3: Add Doppler Ultrasound

• Use color Doppler to identify:
  • Vasa previa (vessels overlying the internal os)
  • Distinguish between fetal and maternal vessels 1, 2
  • Assess for abnormal placental vascularity (if placenta accreta is suspected) 2

Evidence Supporting Transvaginal Approach

Transvaginal ultrasound has demonstrated superior diagnostic accuracy compared to transabdominal ultrasound:

• Provides better visualization of the internal os and its relationship to the placenta 4
• Shows greater clarity of diagnosis, particularly for posterior placentas 3
• Has been shown to correctly identify placental location in 29/34 cases compared to only 16/34 with transabdominal ultrasound 4

Safety Considerations

Despite previous concerns, transvaginal ultrasound is safe in patients with suspected placenta previa:

• Multiple studies have shown no increased risk of bleeding with transvaginal probe placement 3, 4
• Should be performed using real-time imaging as the probe is advanced 1
• The American College of Radiology and American College of Obstetricians and Gynecologists both affirm its safety 2

Follow-up Recommendations

• If placenta is ≥2 cm from the internal os at 18-20 weeks, no follow-up is necessary 1
• If placenta extends over or near the internal os, follow-up examinations are recommended at:
  • 28-30 weeks
  • 32-34 weeks 2

Clinical Pearls and Pitfalls

• Pitfall: Relying solely on transabdominal ultrasound may lead to false-positive diagnoses of placenta previa

  • Transvaginal ultrasound has ruled out placenta previa in cases misdiagnosed by abdominal ultrasound 4

• Pearl: Transrectal ultrasound may be considered as an alternative to transvaginal ultrasound with equivalent quality and potentially less risk of trauma 5

• Caution: When performing transvaginal ultrasound in placenta previa:

  • Use real-time imaging throughout the procedure
  • Ensure proper angle between cervix and vaginal probe
  • Monitor the relationship between probe tip and cervix 1

By following this approach, clinicians can accurately diagnose placenta previa, determine appropriate management, and minimize risks to both mother and fetus.