- Objective Volume measurements by three-dimensional (3D) ultrasonography are considered more accurate than those performed by two-dimensional (2D) ultrasonography. The purpose of this study was to compare the agreement of three techniques, as well as the inter- and intraobserver agreements for volume measurements of fetal fluid-filled structures. Methods Fifty 3D volume datasets of fetal stomachs and bladders were explored. Volume measurements were performed independently by two observers using: (1) Virtual Organ Computer-aided AnaLysis (VOCAL™); (2) inversion mode; and (3) ‘manual segmentation’. Reliability was evaluated using intraclass correlation coefficient (ICC), and Bland–Altman plots were generated to examine bias and agreement. The time required to complete the measurements was compared using Student's t-test or the Wilcoxon Signed Rank Test, and P-values < 0.025 or < 0.05 were considered statistically significant. Results All volume datasets could be measured using the three techniques. A high degree of reliability was observed between: (1) VOCAL and inversion mode (ICC, 0.995; 95% CI, 0.992–0.997); (2) VOCAL and manual segmentation (ICC, 0.997; 95% CI, 0.995–0.998); and (3) inversion mode and manual segmentation (ICC, 0.995; 95% CI, 0.992–0.997). There was good agreement between VOCAL and inversion mode (mean, − 2.4%; 95% limits of agreement, − 20.1 to 15.3%), VOCAL and manual segmentation (mean, − 8.3%; 95% limits of agreement, − 28.8 to 12.2%) as well as between inversion mode and manual segmentation (mean, 5.9%, 95% limits of agreement: − 14.3 to 26%). Manual segmentation and inversion mode measurements were obtained significantly faster than those by VOCAL. Conclusions Volume measurements of fetal fluid-filled structures of relatively regular shape with inversion mode and manual segmentation are feasible. Both techniques have good agreement with VOCAL and are significantly faster than VOCAL. Inversion mode is a reliable method for volume calculations of fluid-filled organs, whereas manual segmentation can be used when volume measurements by VOCAL or inversion mode are technically difficult to obtain, such as solid structures with poorly defined borders as the volume dataset is rotated, like the uterine cervix. Copyright © 2008 ISUOG. Published by John Wiley & Sons, Ltd.