A 28-year-old multigravida at 34 weeks of gestation is referred with a diagnosis of placenta previa made on routine antenatal ultrasound at 20 weeks. She is asymptomatic with no vaginal bleeding. What is the most appropriate investigation to confirm the current status of placenta previa and guide counseling regarding mode of delivery?

## Reassessment of Placenta Previa in Third Trimester **Key Point:** Transvaginal ultrasound (TVS) is the **gold standard** investigation for confirming placenta previa and accurately measuring the distance between the placental edge and the internal cervical os — information that directly guides counseling on mode of delivery. ### Why TVS is Superior to TAS for Placenta Previa Assessment 1. **Accuracy** - TVS has sensitivity >90% and specificity >98% for placenta previa diagnosis - TAS is limited by maternal obesity, posterior placenta, full/empty bladder artefact, and acoustic shadowing from the fetal head - TVS provides precise measurement of the placental edge-to-os distance (in mm), which is the key determinant of delivery mode 2. **Safety of TVS in Suspected Previa** - TVS is **safe** in placenta previa — the transvaginal probe is placed in the vaginal fornix, not through the cervical os - Multiple RCTs and RCOG/ACOG guidelines confirm TVS does not provoke hemorrhage - The concern about TVS causing bleeding in previa is a **common misconception**; it is TAS that is less accurate, not TVS that is unsafe 3. **Clinical Decision Thresholds (RCOG Green-top Guideline No. 27)** - Placental edge **≥20 mm from os** → vaginal delivery can be planned - Placental edge **<20 mm from os** → cesarean section recommended - Placental edge **overlapping os** → major previa; elective LSCS at 37–38 weeks - These precise measurements are only reliably obtained by TVS, not TAS 4. **Role of Cervical Length in Option B** - TVS simultaneously allows cervical length measurement, which provides additional prognostic information regarding preterm labor risk — a clinically relevant co-assessment in a 34-week patient with previa - This makes Option B the most comprehensive and guideline-concordant investigation ### Why the Other Options Are Incorrect | Option | Reason Incorrect | |---|---| | **A) Transperineal US** | Useful when TVS is unavailable; less accurate than TVS for os-to-placenta distance | | **C) Doppler of umbilical artery** | Assesses fetal well-being/growth restriction; does not evaluate placental location | | **D) Repeat TAS at 34 weeks** | Inferior accuracy for placental localization; cannot reliably measure edge-to-os distance | **High-Yield:** RCOG, ACOG, and SOGC guidelines all recommend TVS as the preferred modality for third-trimester reassessment of placenta previa. TAS is the initial screening tool; TVS is the confirmatory gold standard. **Clinical Pearl:** A patient with previa diagnosed at 20 weeks on TAS should have TVS at 32–36 weeks to measure the placental edge-to-os distance precisely. This measurement — not a simple "present/absent" determination — is what guides delivery planning. **Warning:** Do NOT confuse "TVS is contraindicated in previa" with clinical reality. TVS is safe and is the recommended investigation. The probe does not enter the cervix and does not precipitate hemorrhage.