What is the gold standard investigation for diagnosis and classification of placenta previa?

Explanation

## Diagnosis and Imaging of Placenta Previa **Key Point:** Transvaginal ultrasound (TVS) in the third trimester is the gold standard for accurate diagnosis and classification of placenta previa. ### Why Transvaginal Ultrasound? **High-Yield:** Transvaginal ultrasound provides superior resolution of the lower uterine segment and internal cervical os compared to transabdominal imaging. It allows precise measurement of the distance between the placental edge and the internal os, which is essential for classification and counselling. ### Imaging Protocol | Modality | Timing | Accuracy | Clinical Use | |----------|--------|----------|---------------| | **Transabdominal US** | Any trimester | 80–90% | Initial screening; limited for lower segment | | **Transvaginal US** | 3rd trimester (≥28 weeks) | >95% | Gold standard; definitive diagnosis and classification | | **MRI** | Any trimester | >95% | Reserved for specific cases (e.g., placenta accreta suspicion) | | **Abdominal US (1st trim)** | 11–14 weeks | 50–60% | Poor sensitivity; many resolve by 3rd trimester | **Clinical Pearl:** A diagnosis of placenta previa made in the second trimester should be re-evaluated in the third trimester (after 28 weeks) because placental migration occurs in 90% of cases diagnosed early. Only 5–10% of second-trimester previa persists to term. **Warning:** Do NOT perform digital cervical examination in a patient with suspected placenta previa — this can precipitate catastrophic hemorrhage. Diagnosis must be confirmed by imaging first. ### Measurement Technique 1. Identify the internal cervical os 2. Measure the distance from the placental edge to the os 3. Classify based on distance: - Marginal: placental edge at os (0 cm) - Partial: placental edge partially covers os - Complete: placental edge completely covers os - Low-lying: 0–2 cm from os