A 28-year-old multigravida (G3P2) at 28 weeks gestation presents with vaginal bleeding of 150 mL. Ultrasound shows the placenta edge 1.5 cm from the internal cervical os. She is hemodynamically stable. Her previous two deliveries were both by cesarean section (lower segment, classical indication unknown). What is the most important next step in counseling and management?

Explanation

## Clinical Diagnosis: Marginal (Low-Lying) Placenta Previa **Key Point:** Placental edge 1.5 cm from the internal cervical os defines **marginal placenta previa** (also called low-lying placenta). This is distinct from complete previa and carries intermediate hemorrhage risk. ## Classification of Placenta Previa | Type | Definition | Hemorrhage Risk | Vaginal Delivery Possible? | |------|-----------|-----------------|---------------------------| | **Complete (Central)** | Placenta covers entire internal os | Very high | No (rare exceptions) | | **Partial** | Placenta partially covers internal os | High | No | | **Marginal (Low-lying)** | Placental edge 0–2 cm from internal os | Moderate | Possibly, if no recurrent bleeding | | **Low-lying (non-previa)** | Placental edge >2 cm from internal os | Low | Yes | **High-Yield:** At 28 weeks, marginal previa may migrate cephalad (away from the os) in up to 50% of cases as the lower uterine segment develops. However, this patient requires admission because: 1. **Recurrent bleeding risk is real:** Marginal previa carries ~40–50% risk of further bleeding. 2. **Corticosteroids are indicated:** At 28 weeks, fetal lung maturity is critical; betamethasone reduces neonatal mortality by ~30%. 3. **Observation is necessary:** Admit to monitor for recurrent hemorrhage, which would mandate earlier delivery. 4. **Cesarean at 37 weeks is standard:** Even if placenta migrates, the prior two cesarean deliveries increase uterine scar rupture risk with labor; planned repeat cesarean is safer. ## Management Pathway for Marginal Previa ```mermaid flowchart TD A[Marginal placenta previa at 28 weeks]:::outcome --> B[Admit for expectant management]:::action B --> C[Corticosteroids: betamethasone 12 mg IM × 2]:::action C --> D[Bed rest, IV access, cross-match]:::action D --> E{Recurrent bleeding?}:::decision E -->|Yes| F[Cesarean delivery]:::action E -->|No| G[Continue observation until 37 weeks]:::action G --> H[Repeat ultrasound at 32-34 weeks]:::action H --> I{Placenta migrated?}:::decision I -->|Yes, edge >2 cm| J[Reassess for vaginal delivery trial]:::action I -->|No, still marginal| K[Planned cesarean at 37 weeks]:::action ``` **Clinical Pearl:** Placental migration occurs in ~50% of marginal previas diagnosed before 20 weeks, but only ~10% of those diagnosed after 30 weeks. This patient at 28 weeks has intermediate likelihood of migration. ## Why Cesarean at 37 Weeks? **Key Point:** This patient has **two prior cesarean deliveries**. Even if the placenta migrates, she is a candidate for **planned repeat cesarean** rather than TOLAC (trial of labor after cesarean) because: 1. **Uterine scar rupture risk:** Two prior cesareans increase scar rupture risk to ~1–2% (vs. 0.3% for one prior cesarean). 2. **Hemorrhage risk:** Marginal previa with labor increases bleeding risk further. 3. **Guideline recommendation:** ACOG suggests planned repeat cesarean for women with two or more prior cesareans, especially with additional risk factors (e.g., bleeding in current pregnancy). ## Why NOT Vaginal Delivery Now? **Warning:** Recommending vaginal delivery at this stage is premature and unsafe because: - Recurrent bleeding is likely (40–50% of marginal previas bleed again). - Placental migration is uncertain at 28 weeks. - Two prior cesareans make TOLAC riskier than planned repeat cesarean. [cite:Williams Obstetrics 26e Ch 34; ACOG Practice Bulletin #203]