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Purpose To investigate the variability of the normal-sized fetal renal pelvis (≤5 mm) over time and to analyze repeatability of measurements. Materials and Methods 98 fetal renal pelvises and 49 fetal urinary bladders were analyzed at a gestational age of 17–20 weeks at St. Olavs Hospital, Trondheim, Norway. The anterior-posterior diameter (APD) of the fetal renal pelvis and two diameters of the fetal bladder were measured with an interval of at least 30 min. Intra- and interobserver variations and variations over time and in association with bladder size were investigated. Results The mean difference in renal pelvis size between the first and second measurements was 0.09 mm (95% CI, −0.09 to 0.26 mm). The variation over time was ≤1 mm in 85% of cases and the renal pelvis was ≤4 mm in both the first and second examinations in 92% of cases. The intraclass correlation coefficient (ICC) was 0.54 (95% CI: 0.31 to 0.69). We did not observe any association between variation of bladder size and variation of APD. The difference in fetal renal pelvis size was ≤1 mm in 70% of observations for the first examiner and 58% for the second examiner. The intraobserver ICCs were 0.71 (95% CI: 0.62–0.78) and 0.60 (95% CI: 0.50–0.70) for the two observers respectively. The interobserver difference was ≤1 mm in 72% of cases and the interobserver ICC was 0.56 (95% CI: 0.34–0.71). Conclusion The variation of the APD of the fetal renal pelvis over time was small in fetuses with the APD in the lower range and can mainly be explained by intraobserver variation. © Georg Thieme Verlag KG Stuttgart · New York. Published under a Creative Commons Attribution Non-Commercial No Derivatives License.