A 28-year-old multiparous woman at 28 weeks gestation presents with painless vaginal bleeding. Vital signs are stable. On speculum examination, no cervical lesions are seen. Transabdominal ultrasonography shows a low-lying placenta with no retroplacental clot. The bleeding continues despite conservative management. What is the most appropriate next investigation to differentiate between placenta previa and abruptio placentae?

Explanation

## Differentiating Placenta Previa from Abruptio Placentae ### Clinical Presentation Overlap **Key Point:** Both placenta previa and abruptio placentae present with antepartum hemorrhage (APH), but they differ in: - **Placenta previa:** painless bleeding, stable vitals, visible placenta at internal os - **Abruptio placentae:** painful bleeding, uterine tenderness, fetal distress, retroplacental clot on USG In this case, the patient has **painless bleeding** and **stable vitals** (favoring previa), but transabdominal USG is **inconclusive** (no retroplacental clot seen, but low-lying placenta noted). The diagnosis must be clarified. ### Role of Transvaginal Ultrasonography **High-Yield:** Transvaginal USG is the **gold standard** for assessing placental location and ruling out placenta previa: - **Superior resolution** of the lower uterine segment and internal cervical os - Allows **precise measurement** of distance from placental edge to internal os - **Diagnostic criteria for previa:** placental edge ≤20 mm from internal os at 16–24 weeks; ≤10 mm at ≥24 weeks - Safe in the second and third trimester (no increased risk of bleeding) - Confirms or excludes previa, guiding further management **Clinical Pearl:** Transvaginal USG is **contraindicated in confirmed placenta previa** (risk of placental disruption and hemorrhage), but is **safe and indicated** when previa is suspected but not yet confirmed by transabdominal imaging. ### Why Other Options Are Incorrect | Investigation | Why Not Appropriate | |---|---| | **Digital cervical examination** | Absolutely contraindicated if previa is suspected — risk of massive hemorrhage. Never perform without confirming previa is ruled out. | | **Kleihauer–Betke test** | Measures fetal–maternal hemorrhage for Rh prophylaxis; does NOT differentiate between previa and abruption. | | **Repeat transabdominal USG in 2 weeks** | Delays diagnosis and management. Transvaginal USG provides immediate, definitive answer. | **Warning:** **NEVER perform digital cervical examination** in a patient with suspected placenta previa — this is a classic exam trap. Confirm previa is ruled out first (via transvaginal USG) before any digital exploration. ### Management Algorithm ```mermaid flowchart TD A[APH at 28 weeks]:::outcome --> B{Transabdominal USG findings?}:::decision B -->|Previa confirmed| C[Avoid digital exam]:::urgent B -->|Previa ruled out| D[Safe to perform digital exam if needed]:::action B -->|Inconclusive/Low-lying| E[Transvaginal USG]:::action E --> F{Distance from os?}:::decision F -->|≤10 mm| G[Placenta previa confirmed]:::outcome F -->|>10 mm| H[Previa ruled out<br/>Consider abruption]:::outcome ``` [cite:Williams Obstetrics 26e Ch 34] ---