OBJECTIVES:To investigate the intra-and inter-observer variation of the manual 3D Virtual Organ Computer-aided Analysis (VOCAL) in calculating fetal bladder volume and compare results from VOCAL with Sonography-based Automated Volume Count (SonoAVC) and a mathematical calculation based on three radiuses. METHODS:Forty-five women attending the second trimester routine scan at St. Olavs Hospital, Trondheim, Norway were included in a prospective observational study. 3D acquisition of the fetal bladder was performed with Voluson E8 and the results were compared with intra-class correlation coefficient (ICC), repeatability coefficient and limits of agreement. RESULTS:Good intra-observer agreement for both examiners was observed. ICCs were 0.98 and 0.99 and the repeatability coefficients were ± 27 and ± 25 mm 3 . However, we found a significant difference between the two observers. The mean difference was 20 mm 3 (95% CI 9-32). Inter-observer ICC was 0.96. The mean volume based on radiuses was 148 (SD 14) mm 3 vs. 145 (SD 14) mm 3 based on VOCAL and 161 (SD 16) mm 3 based on SonoAVC. The mean volume based on SonoAVC was significant larger than volumes based on radiuses. The ICCs between methods were good and varied from 0.82 to 0.93. The mean time used in calculating volumes was 94 seconds using VOCAL, 86 seconds using SonoAVC and 28 seconds calculating from radiuses.CONCLUSIONS: VOCAL and SonoAVC did not show advantages compared to calculations based on radiuses. Therefore, we recommend calculating volumes of spherical fetal organs from three radiuses because this method was easiest to perform.