Evolving Criteria for Placenta Previa

Most obstetric providers use decades-old terminology to describe placenta previa as complete, partial, or marginal. However, transvaginal sonography (TVS) has yielded a more clinically useful measurement: the distance between the internal cervical os and the placenta. A distance of 20 mm from placental edge to internal cervical os is becoming a new criterion for performing term cesarean delivery in women with placenta previa. In a retrospective study at a single Italian hospital, researchers assessed delivery modes and outcomes of singleton pregnancies among women with placenta previa that was detected with TVS <28 days before delivery. Placenta previa was diagnosed if the placental edge was within 20 mm of the internal os or overlapped it. Labor was allowed in women who had placental edges from 1 mm to 20 mm of the internal os, cephalic presentation, and no prior cesarean deliveries.

In 24 women (group 1), the placental edge was from 1 mm to 10 mm of the os; in 29 women (group 2), the distance was 11 mm to 20 mm. Rates of prelabor bleeding were higher in group 1 than in group 2 (29% vs. 3%; odds ratio, 11.5; 95% confidence interval, 1.6–76.7) as were rates of cesarean delivery (75% vs. 31%; OR, 6.7; 95% CI, 2.0–22.0). Rates of postpartum hemorrhage were 21% and 10%, respectively, in groups 1 and 2; these differences did not reach statistical significance. More than two thirds of placenta previa patients with distances of 11 mm to 20 mm delivered vaginally. Thus, a distance of $≥$ 11 mm between the placenta and the os predicted lower risk for bleeding before and during labor as well as greater likelihood of vaginal birth.

Comment: Based on their reevaluation of the 20-mm placenta-to-os distance for cesarean delivery in placenta previa patients, the authors advocate a new threshold of $≥$ 11 mm as a safe cutoff that allows a woman to advance to a trial of labor, and thereby avoid unnecessary cesarean delivery. Although the thresholds described in this study might help improve safety outcomes for women with placenta previa, confirming them in larger obstetric populations would be appropriate.

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