Persistence of placenta previa according to gestational age at ultrasound detection

OBJECTIVE: To evaluate gestational age at ultrasound detection of placenta previa as a predictor of previa persistence until delivery, and to estimate the effects of previa type, parity, and prior cesarean delivery on previa persistence. METHODS: This was a retrospective cohort study of pregnancies with placenta previa detected during transabdominal or endovaginal ultrasound examination. Previa was categorized as complete if the placenta completely covered the internal cervical os or incomplete if the inferior placental edge partially covered or reached the margin of the os. Gestational age was grouped into 4-week intervals from 15 to 36 weeks. The outcome was cesarean delivery for persistent previa. RESULTS: Previa was detected during 940 ultrasound examinations in 714 pregnancies. Of those with placenta previa at 15-19 weeks, 20-23 weeks, 24-27 weeks, 28-31 weeks, and 32-35 weeks, previa persisted until delivery in 12%, 34%, 49%, 62%, and 73%, respectively. At each interval, complete previa was more likely to persist than incomplete previa, all P <.001. Prior cesarean delivery was an independent risk factor for persistent previa among women diagnosed with previa in the second trimester, P <.05. However, parity was not an independent risk factor for persistence at any gestational age interval after adjusting for prior cesarean delivery. CONCLUSION: Gestational age at ultrasound detection of placenta previa may be used to predict likelihood of previa persistence. After midpregnancy, risk of persistence appears to be higher than previously reported. Type of placentation and prior cesarean delivery are important factors that modify the risk that previa will complicate delivery.