Reproducibility Study of Fetal 3-D Volumetry in the First Trimester: Effect of Fetal Size and Rotational Angle of VOCAL Software

Intra- and inter-observer reproducibility of fetal volume measurement by 3-D ultrasound scan (using VOCAL [Virtual Organ Computer-Aided Analysis] software) in 27 fetuses at 7 to 13 wk was studied. For intra-observer variability, the mean difference (MD) and 95% limits of agreement (95% LOA) at 12°, 18° and 30° were MD12 = 0.097, 95% LOA12 = -0.87 to +1.06; MD18 = 0.07, 95% LOA18 = -1.31 to +1.45; and MD30 = -0.07, 95% LOA30 = -1.55 to +1.41. The standard deviation of the differences (SDDIF) increased with crown-rump length at 12° (p = 0.0016), 18° (p = 0.0011) and 30° (p = 0.02). For inter-observer variability, MD12 = 0.15, 95% LOA12 = -1.65 to +1.95; MD18 = 0.042, 95% LOA18 = -1.79 to +1.87; and MD30 = 0.19, 95% LOA30 = -1.24 to +1.62. SDDIF increased with crown-rump length at 18° (p = 0.0084) and 30° (p = 0.0073). The accuracy of fetal volume measurement was not influenced by rotational angle or fetal size. Precision deteriorated for wider rotational angles and larger fetuses.