Objective: To establish prognostic data regarding fetal hydronephrosis using the anteroposterior diameter (APD) and the need for interventional surgery in the Korean population. Methods: A total of 187 children with an APD of <4 mm on obstetric ultrasound scans at any gestational age were retrospectively reviewed. The affected renal units were divided into 2 groups: surgical and nonsurgical. The ultrasound findings were compared at 3 gestational ages: second trimester (15-26 weeks' gestation), early third trimester (27-33 weeks' gestation), and late third trimester (34-40 weeks' gestation). Results: The area under the receiver operating characteristic curve was 0.770, 0.828, and 0.812 at the second, early third, and late third trimesters, respectively. A 100% sensitivity for predicting postnatal surgery could be achieved at a cutoff APD of 5 mm during the second trimester, 8 mm during the early third trimester, and 10 mm during the late third trimester if scheduled antenatal ultrasound scans were performed. A cutoff APD of 11 mm during the second trimester was of diagnostic value in selecting children at risk of postnatal surgery with an odds ratio of 5.13 (95% confidence interval 1.62-16.25), with relatively high sensitivity and specificity. With a cutoff of 15 mm during the early third and late third trimesters, the odds ratio was 11.51 (95% confidence interval 5.05-26.23) and 6.94 (95% confidence interval 3.30-14.57), respectively. Conclusion: Compared with an APD of 10 mm, the most commonly used standard cutoff value in predicting postnatal hydronephrosis and its outcome, an APD cutoff of 5, 8, and 10 mm during the second, early third, and late third trimesters, respectively, is more specific in predicting the need for postnatal surgical intervention in the Korean population.