Purpose. It was to quantify the intraobserver and interobserver variability of the sonographic measurements of renal pelvis and classify hydronephrosis severity. Methods. Two ultrasonographers evaluated 17 fetuses from 23 to 39 weeks of gestation. Renal pelvis APD were taken in 50 renal units. For intraobserver error, one of them performed three sequential measurements. The mean and standard deviation from the absolute and percentage differences between measurements were calculated. Bland-Altman plots were used to visually assess the relationship between the precision of repeated measurements. Hydronephrosis was classified as mild (5.0 to 9.9？mm), moderate (10.0 to 14.9？mm), or severe (≥15.0？mm). Interrater agreement were obtained using the Kappa index. Results. Absolute intraobserver variation in APD measurements was %. Interobserver variation of ultrasonographers was %. Neither intraobserver or interobserver error increased with increasing APD size. The overall percentage of agreement with the antenatal hydronephrosis diagnosis was 64%. Cohen's Kappa to hydronephrosis severity was 0.51 (95% CI, 0.33 to 0.69). Conclusion. Inter and intraobserver APD measurement errors were low in these group, but the agreement to hydronephrosis diagnosis and classification was fair. We suggest that standard and serial APD measurement can better define and evaluate fetal hydronephrosis. 1. Introduction The advent of routine antenatal ultrasonography has allowed for an appreciation of the true incidence of urological abnormalities and has identified many patients who require reassessment postnatally [1]. In spite of such advances, however, the issue of antenatal hydronephrosis remains a common and challenging problem, with postnatal influences [2, 3]. There have been a number of studies assessing the accuracy of fetal renal pelvic dilatation (RPD) as an indicator of urinary tract anomalies [4–8]. The single most widely used parameter is the anteroposterior diameter (APD) of the renal pelvis, a simple parameter whose application is now widespread in prenatal diagnostics [9]. However, the reproducibility measurement of this parameter has scarcely been investigated. Furthermore, during routine ultrasound examinations, the size of the renal pelvis varies considerably over time [10]. Though the renal collecting system can be influenced by physiological conditions (maternal hydration and degree of bladder filling [11, 12]), the lack of a full technical description and validation of that measurement seems to be a central factor. The aim of our investigation was to evaluate the
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