A 28-year-old multigravida (G3P2) at 30 weeks gestation presents with painless vaginal bleeding. Transvaginal ultrasound confirms placenta previa. Which of the following is the most common predisposing factor for the development of placenta previa in this patient?

Explanation

## Risk Factors for Placenta Previa ### Definition of Predisposing Factors Placenta previa results from abnormal trophoblastic invasion and poor endometrial decidualization. Multiple maternal and obstetric factors increase this risk, with uterine instrumentation being the most significant. ### Most Common Predisposing Factor: Previous Uterine Instrumentation **Key Point:** Previous uterine curettage (D&C) or cesarean delivery is the commonest predisposing factor for placenta previa. This occurs because: 1. Instrumentation damages the endometrium and reduces decidualization 2. Scarring and fibrosis alter normal placental implantation 3. Abnormal vascularity in scarred areas promotes trophoblastic invasion 4. The lower segment is particularly vulnerable to scarring from cesarean delivery ### Comparative Risk Factors | Risk Factor | Relative Risk | Mechanism | Frequency | |-------------|---------------|-----------|----------| | Previous cesarean delivery | 2–4× | Endometrial scarring, abnormal vascularity | Most common | | Previous D&C/curettage | 2–3× | Endometrial damage, reduced decidualization | Common | | Maternal age >35 years | 1.5–2× | Reduced endometrial receptivity, aging placenta | Moderate | | Maternal smoking | 1.5–2× | Reduced placental perfusion, endothelial dysfunction | Moderate | | Maternal hypertension | 1.2–1.5× | Placental vascular disease | Least common | ### Pathophysiology of Instrumentation-Related Previa **High-Yield:** Uterine curettage and cesarean delivery cause: 1. **Endometrial damage:** Loss of normal basalis layer regeneration 2. **Asherman's syndrome risk:** Intrauterine adhesions reduce available implantation sites 3. **Abnormal vascularity:** Neovascularization in scarred areas 4. **Trophoblastic invasion:** Abnormal placentation in areas of poor decidualization 5. **Lower segment vulnerability:** The lower segment is most affected by cesarean scars ### Clinical Correlation **Clinical Pearl:** The risk of placenta previa increases with: - Number of previous cesarean deliveries (cumulative risk) - Number of previous D&C procedures - Time interval since instrumentation (risk persists across pregnancies) - Presence of intrauterine adhesions on hysteroscopy **Mnemonic — Risk Factors for Placenta Previa (CHASM):** - **C**esarean delivery / Curettage (most common) - **H**ypertension (maternal) - **A**dvanced maternal age - **S**moking (maternal) - **M**ultiparity (relative protection, but increases absolute numbers) ### Why Other Options Are Less Common - **Maternal age >35 years:** While advanced maternal age increases risk, it is less common than previous instrumentation as a primary factor - **Maternal smoking:** Increases risk but is less frequent than instrumentation-related damage - **Maternal hypertension:** Has the lowest relative risk among the listed options [cite:Williams Obstetrics 26e Ch 34; Obstetric and Gynaecological Society of India Guidelines on APH]