• First trimester;
  • gestational sac;
  • volume measurement;
  • three-dimensional imaging;


Objective. To compare different three-dimensional (3D) methods in the assessment of gestational sac volume (GSV). Design. Cross-sectional study involving 74 normal pregnancies between 7 and 11 weeks. Setting. Department of Obstetrics, São Paulo Federal University (UNIFESP). Methods. GSV was measured through 3D ultrasound using the planimetric, virtual organ computer-aided analysis (VOCAL) and extended imaging virtual organ computer-aided analysis (XI VOCAL) methods. The planimetric method used a sequence of adjacent planes that are 3 mm thick. For the VOCAL methods, six adjacent planes and a 30° rotation were used. A total of 15 adjacent planes were used for the XI VOCAL method. Regression models with a determination coefficient (R2) were created to assess the correlation between GSV and gestational age (GA). Intraclass correlation coefficient (ICC) and Bland-Altman graphs were used to assess the correlation between the three methods and ANOVA was used to compare means. Results. All three methods showed a correlation between GSV and GA (R2 = 0.65 for XI VOCAL, R2 = 0.65 for planimetric and R2 = 0.66 for VOCAL). There was a strong correlation between the three methods (XI VOCAL vs. planimetric ICC = 0.995; XI VOCAL vs. VOCAL ICC = 0.998 and planimetric vs. VOCAL ICC = 0.995) without any significant differences according to the Bland-Altman graphs or ANOVA (p < 0.002). Conclusions. The three 3D ultrasound methods used for GSV assessment between 7 and 11 weeks are concordant. These methods can be used interchangeably during the first trimester of pregnancy to measure GSV.