The use of second-trimester transvaginal sonography to predict placenta previa.

Our objective was to determine the incidence and rate of persistence of placenta previa diagnosed at 15-20 weeks' gestation by using transvaginal sonography (TVS), and to describe the characteristics of TVS that predict placenta previa at delivery. Patients having placental tissue within 20 mm of the cervical os were prospectively identified by transabdominal ultrasound and underwent TVS. The distance of the placental edge from the cervical os was measured in millimeters. Characteristics of TVS predicting placenta previa at delivery were analyzed by logistic regression. The incidence of placenta previa diagnosed by TVS at 15-20 weeks was 1.1%; 14% persisted until delivery. Gestational age at the time of TVS and the distance of the placental edge to the cervical os helped predict placenta previa at delivery. Between 15 and 24 weeks' gestation, placenta overlapping the internal os by > or = 10 mm identified patients at risk of placenta previa at delivery with 100% sensitivity and 85% specificity. The use of TVS in the second trimester to diagnose placenta previa resulted in a lower incidence than was historically reported with the use of transabdominal ultrasound. The distance of the placental edge from the cervical os helps identify patients at risk of previa at delivery.