Abstract: Many cases of placenta previa diagnosed during the early second-trimester have migrated toward the uterine fundus，but a few of them are still present at third-trimester and/or delivery. We name it as persistent placenta previa（PPP）. Lots of index can used to predict PPP in second-trimester，such as：os-placenta distance，the low placental edge thickness，placenta localization，the specificity of ultrasound images，etc. But in these index，the os-placenta distance is the most convenient，accurate and feasible. Over all，the single index have limited value in predicting PPP，but the prediction is not much valuable. At present，the model which is combined with a number of indicators to predict PPP is insufficient. In this paper，we assess the predict value of PPP with a single index by ultrasonography，and prospect for the establishment of prediction model with multinomial index for predicting PPP.

Key words: Placenta previa, Ultrasonography, Forecasting, Pregnancy trimester, first, Pregnancy trimester