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To define a better cut-off value of the renal pelvis anteroposterior diameter (RPAPD) to predict persistent or progressive pyelectasis during pregnancy.We retrospectively reviewed 8873 women whose fetal RPAPD was measured. Midtrimester pyelectasis was defined as a RPAPD of ≥4 mm. Persistent/progressive pyelectasis was defined as a RPAPD of ≥10 mm before delivery. A RPAPD cut-off value to predict a persistent/progressive pyelectasis was determined by receiver operating characteristic curve analysis.Among 249 isolated cases of pyelectasis, persistent/progressive pyelectasis was found in 6.9% before delivery. The midtrimester RPAPD cut-off value that best predicted persistent/progressive pyelectasis before delivery was ≥6 mm with sensitivity, specificity, positive and negative predictive values of 64.3%, 88.7%, 30.0%, and 97.1%, respectively.Although most cases of midtrimester fetal pyelectasis regress to normal during pregnancy, those with a RPAPD of ≥6 mm in the midtrimester are at higher risk for persistent or progressive pyelectasis.