What is the recommended initial imaging modality for evaluating suspected placenta previa?

Transvaginal Ultrasound for Evaluation of Placenta Previa

Transvaginal ultrasound should accompany transabdominal ultrasound for the evaluation of suspected placenta previa, as it provides superior visualization of the placental-cervical relationship and is safe to perform despite previous concerns about provoking bleeding. 1

Initial Imaging Approach

Primary Evaluation

• Begin with transabdominal ultrasound for initial assessment
• Follow with transvaginal ultrasound for detailed evaluation, especially when:
  • The placenta is located in the lower uterine segment
  • Detailed assessment of placental-cervical relationship is needed
  • Transabdominal images are inadequate or inconclusive 1, 2

Safety Considerations

• Contrary to historical concerns, transvaginal ultrasound is safe in patients with placenta previa
• Multiple studies have demonstrated no increased risk of bleeding with transvaginal probe placement 3, 4
• Placenta previa is not a contraindication to transvaginal ultrasonography 1

Diagnostic Advantages of Transvaginal Ultrasound

Superior Visualization

• Provides high-resolution images of the placental-myometrial interface 1
• Particularly valuable for posterior placenta previa, where fetal parts often shadow the view on transabdominal ultrasound 4
• Allows more accurate measurement of the distance from placental edge to internal cervical os 5

Diagnostic Accuracy

• Sensitivity of 95% (compared to 86% for transabdominal ultrasound) 6
• Specificity of 93% for both methods 6
• Overall accuracy of 94% (compared to 89% for transabdominal ultrasound) 6
• Can change diagnosis in approximately 23% of cases when added to transabdominal ultrasound 6

Clinical Protocol for Suspected Placenta Previa

Timing of Ultrasound Examinations

• Initial evaluation when placenta previa is suspected (typically during mid-pregnancy anatomy scan)
• Follow-up examinations recommended at:
  • 28-30 weeks
  • 32-34 weeks 1
• This schedule allows assessment of:
  • Potential placenta previa resolution
  • Placental location for delivery planning
  • Possible bladder invasion in cases of placenta accreta spectrum 1

Risk Assessment

• When placenta extends ≥15mm over the internal cervical os at 18-23 weeks, positive predictive value for previa at delivery is 19% with 100% sensitivity 5
• When placenta extends ≥25mm over the internal cervical os, positive predictive value increases to 40% with 80% sensitivity 5

Special Considerations

Placenta Accreta Spectrum Detection

• For patients with risk factors (prior cesarean delivery, placenta previa):
  • Transvaginal ultrasound provides detailed evaluation of placental-myometrial interface 1
  • Color Doppler should be added to assess for abnormal placental vascularity 1

Referral Recommendations

• Women with confirmed placenta previa should be referred to a level III or IV center with experience in managing these cases 1
• This is particularly important when placenta accreta spectrum is suspected 1

Common Pitfalls to Avoid

• Avoiding transvaginal ultrasound due to fear of provoking bleeding - multiple studies confirm its safety 3, 4
• Relying solely on transabdominal ultrasound - may miss or misdiagnose placenta previa, especially with posterior placentas 4
• Performing digital vaginal examination before excluding placenta previa by ultrasound 2
• Inadequate follow-up - placenta previa diagnosed early in pregnancy requires serial ultrasound evaluation as "migration" can occur with uterine growth 1, 5

By following this evidence-based approach to evaluating suspected placenta previa with transvaginal ultrasound, clinicians can optimize diagnostic accuracy while maintaining patient safety, ultimately reducing maternal and fetal morbidity and mortality.