• first trimester;
  • sac volume;
  • three-dimensional ultrasonography;



Our main objective was to determine the value of three-dimensional ultrasonography (3DUS) and Virtual Organ Computer-aided AnaLysis (VOCAL) in the evaluation of gestational sac volume and morphology during early pregnancy.


Twenty-eight normal early pregnancies were scanned approximately every 2 weeks using transabdominal (TAS) and transvaginal (TVS) sonography. The VOCAL technique was used to create computerized surface models to classify gestational sac shapes as discoid or ellipsoid. Serial sac volume changes were analyzed using repeated measures ANOVA. Bland–Altman plots determined examiner bias and limits of agreement (LOA) for sac volume measurements. Gestational sac volumes were compared between the two-dimensional (2D) ellipsoid and VOCAL techniques. Differences between volume measurements were tested using the two-tailed paired t-test with statistical significance at the P < 0.05 level.


Each subject was examined at a mean ± SD menstrual age of 7.9 ± 0.6 weeks (Scan 1), 9.9 ± 0.6 weeks (Scan 2), and 11.9 ± 0.6 weeks (Scan 3). Sac volumes significantly increased over time from 22 ± 11 mL at Scan 1, to 57 ± 21 mL at Scan 2 and 116 ± 35 mL at Scan 3 (P < 0.001). Predominant sac shapes were classified as ellipsoid (76.2%) or discoid (23.8%). Additional descriptors included: concave (60.7%), irregular (53.6%), or smooth (7.1%), with 19% of the overall group having more than one additional shape attribute. Clinically acceptable volume measurement bias and agreement were found for the following comparisons: (1) TAS versus TVS; (2) interobserver volume measurements; and (3) intraobserver volume measurements. The VOCAL technique yielded slightly greater sac volumes (64 ± 45.4 mL) when compared to the 2D ellipsoid model (48.6 ± 36.8 mL) (28.9 ± 24.3% (95% limit of agreement range, − 18.7 to 76.5%), P < 0.001).


Reproducible sac volume measurements can be obtained using VOCAL with either TAS or TVS. Early gestational sacs variably appear as discoid or ellipsoid structures with a concave indentation from the placenta. Sac volumes can be underestimated if an ellipsoid shape is assumed. Morphological and quantitative analysis of the gestational sac may provide baseline parameters for studying patients at risk for early pregnancy failure. Copyright © 2006 ISUOG. Published by John Wiley & Sons, Ltd.