A 32-year-old primigravida at 38 weeks gestation presents with sudden onset of painless vaginal bleeding. On examination, the uterus is soft and non-tender. Fetal heart rate is 140 bpm. Which is the investigation of choice to confirm the diagnosis before proceeding to cesarean section?

Explanation

## Clinical Scenario Painless vaginal bleeding in the third trimester with a soft, non-tender uterus and reassuring fetal heart rate is highly suggestive of placenta previa. The clinical presentation mandates confirmation before any intervention. ## Investigation of Choice: Speculum Examination Followed by Transabdominal Ultrasound **Key Point:** In suspected placenta previa, digital cervical examination is absolutely contraindicated as it can precipitate massive hemorrhage. Speculum examination is safe and allows visualization of the bleeding source. **High-Yield:** Transabdominal ultrasound is the gold standard for confirming placenta previa. It accurately determines the distance between the placental edge and the internal cervical os: - Placenta previa: placental edge ≤ 2 cm from internal os - Marginal previa: 2–3 cm - Low-lying placenta: > 3 cm **Clinical Pearl:** Transvaginal ultrasound is contraindicated in suspected placenta previa due to the risk of hemorrhage; transabdominal approach is safe and diagnostic. ## Why This Approach Works | Step | Rationale | |------|----------| | Speculum exam | Safe visualization; rules out cervicitis, cervical polyps, or cervical bleeding | | Transabdominal ultrasound | Confirms placental location; guides management (expectant vs. cesarean) | | Avoids digital cervical exam | Prevents catastrophic hemorrhage if previa is present | **Mnemonic:** **SAFE** — Speculum, Avoid digital exam, Fetal monitoring, Evaluate with ultrasound (transabdominal). [cite:Williams Obstetrics 25e Ch 34]