Objective: To clarify whether distinguishing between the uterine isthmus and cervix can improve the accuracy of diagnosing placenta previa at term. Methods: A multicenter prospective observational study was conducted among pregnant women with suspected placenta previa at 20-24 weeks' gestation. Subjects were divided into the open isthmus group and closed isthmus group. The accuracy of diagnosing placenta previa at term was compared between the 2 groups. Results: We screened 9,341 patients, and 53 (0.6%) met the inclusion criteria. Nineteen cases with an open isthmus and 34 with a closed isthmus were followed. The accuracy for diagnosing placenta previa or a low-lying placenta at term was 94.7% in the open isthmus group and 26.5% in the closed isthmus group (p < 0.001). Elective or emergency Cesarean section was required in 100% of cases in the open isthmus group and 20.6% in the closed isthmus group (p < 0.001). Conclusion: A high prediction rate of placenta previa was obtained by using transvaginal ultrasound at 20-24 weeks' gestation after the isthmus opened by carefully distinguishing between the cervix and isthmus.
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