## Classification of Placenta Previa **Key Point:** Marginal placenta previa (also called edge placenta previa) is the most common type, accounting for approximately 40–50% of all placenta previa cases. ### Frequency Distribution | Type | Frequency | Definition | |------|-----------|------------| | **Marginal** | 40–50% (Most common) | Placental edge reaches the internal cervical os but does not cover it | | **Partial** | 25–30% | Placenta partially covers the internal cervical os | | **Complete** | 20–25% | Placenta completely covers the internal cervical os | | **Low-lying** | Variable | Placenta is in lower uterine segment but does not reach the os | ### Clinical Significance **High-Yield:** Marginal previa has the best prognosis for vaginal delivery if labour occurs after 37 weeks, because the placental edge may move away from the cervix as the lower uterine segment stretches during labour and descent of the presenting part. **Clinical Pearl:** Many cases of low-lying placenta and marginal previa diagnosed in the second trimester "resolve" by term due to differential growth of the lower uterine segment — a phenomenon called placental "migration" (actually upward movement relative to the cervix, not true migration). ### Management Implications - **Marginal previa:** Vaginal delivery may be attempted if no bleeding after 37 weeks and cephalic presentation - **Partial/Complete previa:** Elective caesarean section at 38–39 weeks is standard - **Low-lying:** Managed expectantly; delivery route depends on position at term **Warning:** Do not confuse "low-lying placenta" with placenta previa — low-lying is NOT previa by definition, as the placental edge does not reach the internal os.
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