Abstract: Volumetric segmentation of the placenta using 3-D ultrasound is currently performed clinically to investigate correlation between organ volume and fetal outcome or pathology Previously, interpolative or semi-automatic contour-based methodologies were used to provide volumetric results We describe the validation of an original random walker (RW)-based algorithm against manual segmentation and an existing semi-automated method, virtual organ computer-aided analysis (VOCAL), using initialization time, inter- and intra-observer variability of volumetric measurements and quantification accuracy (with respect to manual segmentation) as metrics of success Both semi-automatic methods require initialization Therefore, the first experiment compared initialization times Initialization was timed by one observer using 20 subjects This revealed significant differences (p < 0001) in time taken to initialize the VOCAL method compared with the RW method In the second experiment, 10 subjects were used to analyze intra-/inter-observer variability between two observers Bland-Altman plots were used to analyze variability combined with intra- and inter-observer variability measured by intra-class correlation coefficients, which were reported for all three methods Intra-class correlation coefficient values for intra-observer variability were higher for the RW method than for VOCAL, and both were similar to manual segmentation Inter-observer variability was 094 (088, 097), 091 (081, 095) and 080 (061, 090) for manual, RW and VOCAL, respectively Finally, a third observer with no prior ultrasound experience was introduced and volumetric differences from manual segmentation were reported Dice similarity coefficients for observers 1, 2 and 3 were respectively 084 ± 012, 094 ± 008 and 084 ± 011, and the mean was 087 ± 013 The RW algorithm was found to provide results concordant with those for manual segmentation and to outperform VOCAL in aspects of observer reliability The training of an additional untrained observer was investigated, and results revealed that with the appropriate initialization protocol, results for observers with varying levels of experience were concordant We found that with appropriate training, the RW method can be used for fast, repeatable 3-D measurement of placental volume