## Classification of Placenta Previa **Key Point:** Placenta previa is classified based on the relationship between the placental edge and the internal cervical os. ### Types and Frequency | Type | Definition | Frequency | Clinical Significance | |------|-----------|-----------|----------------------| | **Marginal** | Placental edge reaches internal os but does not cover it | 40–50% | Most common; lowest bleeding risk | | **Partial** | Placental edge partially covers internal os | 30–40% | Intermediate risk | | **Complete** | Placental edge completely covers internal os | 10–20% | Highest bleeding risk; vaginal delivery contraindicated | | **Low-lying** | Placental edge 0–2 cm from internal os but does not reach it | Not true previa | Often resolves with placental migration | **High-Yield:** Marginal placenta previa is the most common type and carries the lowest perinatal morbidity and mortality because the placental edge does not encroach significantly on the cervical opening. ### Clinical Implications **Clinical Pearl:** The distinction between marginal and partial previa is critical for counselling regarding mode of delivery and risk of antepartum hemorrhage. Marginal previa may permit vaginal delivery if labour is induced in a controlled setting, whereas partial and complete previa require planned cesarean delivery. **Tip:** On transvaginal ultrasound, measure the distance from the placental edge to the internal cervical os. If ≥2 cm, the patient is not at risk for previa-related hemorrhage in labour.
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