A 32-year-old primigravida at 28 weeks of gestation presents to the antenatal clinic with painless vaginal bleeding. On speculum examination, no cervical source is identified. What is the investigation of choice to confirm the diagnosis of placenta previa?

Explanation

## Investigation of Choice for Placenta Previa Diagnosis ### Transvaginal Ultrasound — Gold Standard **Key Point:** Transvaginal ultrasound (TVS) is the **investigation of choice** (gold standard) for confirming the diagnosis of placenta previa. It provides superior visualization of the relationship between the placental edge and the internal cervical os compared to transabdominal ultrasound. **High-Yield:** TVS advantages in placenta previa: - Directly visualizes the internal cervical os and placental edge with high resolution - More accurate than TAS, especially when the placenta is posterior or when the bladder is not adequately filled - Overcomes limitations of transabdominal scanning (obesity, overlying bowel gas, posterior placenta) - Allows precise measurement of the distance between placental edge and internal os - Has been shown in multiple studies to have sensitivity and specificity approaching 100% ### Safety of Transvaginal Ultrasound in Placenta Previa **Clinical Pearl:** Contrary to older teaching, transvaginal ultrasound is **NOT contraindicated** in suspected placenta previa when performed carefully. The probe is placed in the anterior fornix and does not reach the internal os; it does not disturb the placenta. Multiple studies (including RCOG guidelines) confirm TVS is safe in this setting. | Feature | Transvaginal (TVS) | Transabdominal (TAS) | |---------|---|---| | **Accuracy** | **Gold standard; ~100% sensitivity/specificity** | Good screening tool; ~95% sensitivity | | **Posterior placenta** | Excellent visualization | Limited | | **Internal os visualization** | Direct, high-resolution | Indirect; requires full bladder | | **Safety** | Safe when performed carefully | Safe | | **Role** | **Confirmatory / gold standard** | Initial screening | ### Why Not Transabdominal Ultrasound? Transabdominal ultrasound is the **initial/screening** investigation and is appropriate as a first step. However, it has limitations: - Requires a full bladder for adequate views - Poor visualization of posterior placenta - May over- or under-diagnose previa due to bladder distension artifact - TVS is recommended to **confirm** the diagnosis when TAS is inconclusive or to precisely measure placental-os distance Per **RCOG Green-top Guideline No. 27** and **Williams Obstetrics (25th ed.)**: TVS is the recommended modality for confirming placenta previa, with TAS used as the initial screen. ### Why Not MRI or Digital Examination? - **MRI pelvis**: Reserved for problem-solving (e.g., suspected placenta accreta spectrum); not first-line for previa diagnosis - **Digital cervical examination**: **Absolutely contraindicated** in placenta previa — can precipitate life-threatening hemorrhage by disrupting the placental edge **High-Yield:** The classic teaching sequence: TAS for screening → TVS for **confirmation** (gold standard). Digital examination is always contraindicated. [cite: Williams Obstetrics 25e; RCOG Green-top Guideline No. 27 (Placenta Praevia)]