242 The introduction of routine fetal ultrasonographic examination has improved the detection rate of fetal anomalies; the technical developments of ultrasound equipment have increased diagnostic accuracy. As a con- sequence, congenital anomalies are diagnosed earlier and in a greater number of patients. The most common sonographic abnormality found in the fetal urinary tract is collecting system dilatation. This finding, which may be recognized from the beginning of the second trimester of pregnancy, was assumed in early reports to be physiologic or nonobstructive; therefore, follow-up data were not al- ways obtained after the initial identification. 1,2 Numerous renal pelvis measurements and various gestational age cutoff points have been recommended in the assessment of obstructive fetal uropathy. As a result, renal pelvic mea- surements are not well standardized, which may result in conflicting prenatal ultrasound diagnoses and unclear postnatal follow-up treatment. 3 Furthermore, there are very few data on the outcome of minor degrees of ante- natal renal pelvis dilatation, which was evaluated prospec- tively in an unselected population. Therefore, we conducted a prospective investigation in two hospitals in Brussels (without bias in referral pattern) to determine the exact incidence of urinary tract dilatation that could be detected by antenatal ultrasonography and the value of mild or moderate fetal pelvis dilatation in the predic- tion of significant uropathy. Methods The study was conducted over a 24-month period, dur- ing which there were 5643 births. Prenatal ultrasound scanning was performed as part of a routine evaluation of pregnancy, but with special attention to the urinary tract. Prenatal ultrasound scanning was performed in all fe- tuses once every trimester. The criteria reported by Corteville et al 4 were adopted for the present study: all in- fants whose routine antenatal ultrasound scans showed an anteroposterior renal pelvic diameter of ≥4 mm in the second trimester and/or ≥7 mm in the third trimester From the Department of Perinatal and Pediatric Nephrology, Hôpital Universitaire des Enfants—Reine Fabiola and Hôpital Erasme, a the De- partment of Obstetrics, Hôpital Erasme, b the Department of Obstetrics, Centre Hospitalier Etterbeek-Ixelles, c the Department of Neonatology, Hôpital Erasme, d and the Department of Pediatric Radiology, Hôpital Universitaire des Enfants—Reine Fabiola and Hôpital Erasme, e Uni- versité Libre de Bruxelles. Presented in part at the 12th Congress of the International Pediatric Nephrology Association, Seattle, Wash, September 1-5, 2001. Received for publication January 30, 2002; revised August 6, 2002; ac- cepted August 13, 2002. The members of the Brussels Free University Perinatal Nephrology study group are listed in the Appendix. Reprint requests: Khalid Ismaili, MD, Hôpital Universitaire des En- fants—Reine Fabiola, Department of Perinatal and Pediatric Nephrol- ogy, 15, Avenue J.J. CROCQ, 1020—Brussels, Belgium. E-mail: k.ismaili@yucom.be © 2003, Mosby, Inc. All rights reserved. 0002-9378/2003 $30.00 + 0 doi:10.1067/mob.2003.81 Results of systematic screening for minor degrees of fetal renal pelvis dilatation in an unselected population Khalid Ismaili, MD, a Michelle Hall, MD, a Catherine Donner, MD, PhD, b Dominique Thomas, MD, c Danièle Vermeylen, MD, d and Fred E. Avni, MD, PhD e Brussels, Belgium OBJECTIVE: The purpose of this study was to determine the incidence of minor degrees of renal pelvis di- latation that is detected by antenatal ultrasound scanning in an unselected population and its value in the prediction of significant uropathies. STUDY DESIGN: This prospective study was conducted over a 24-month period. Infants with an anteropos- terior pelvic diameter of 4 mm in the second trimester and/or 7 mm but 15 mm in the third trimester were enrolled. RESULTS: Pyelectasis was found in 4.5% of 5643 fetuses (1.5% with significant uropathy). Among the 213 infants whose cases were followed, 132 infants (62%) had renal anomalies, but only 83 infants (39%) had significant uropathies. The ability of the third-trimester renal pelvis dilatation to predict renal abnormalities showed a positive predictive value of 69%. Pyelectasis that was detected only in the second trimester re- vealed a significant uropathy in 12% of the infants. CONCLUSION: Pyelectasis was found in 4.5% of fetuses. The third-trimester anteroposterior renal pelvis di- ameter of ≥7 mm was the best ultrasound criterion to predict postnatal uropathies. (Am J Obstet Gynecol 2003;188:242-6.) Key words: Prenatal diagnosis, fetal hydronephrosis, urinary tract, ultrasonography, infant