Comparative evaluation of manual and VOCAL methods for amniotic sac volume measurement in early pregnancy

To establish reference intervals for amniotic sac volume (ASV) in early pregnancy (7-12 weeks) and compare the accuracy, reproducibility, and clinical utility of manual and VOCAL ultrasound methods.

This prospective observational study involved 68 singleton pregnancies from two maternal-fetal medicine centers. ASV was measured using manual tracing and virtual organ computer-aided analysis (VOCAL). Regression analysis was performed to determine reference intervals and assess correlations with gestational age and crown-rump length (CRL). Measurement accuracy was evaluated using mean absolute error (MAE) and root mean square error (RMSE), while inter- and intraobserver variability were assessed using intraclass correlation coefficients (ICC) and Bland-Altman analysis.

ASV measurements demonstrated strong correlations with both gestational age and CRL (R²=0.999, p<0.001). VOCAL yielded higher accuracy (MAE: 0.35 cc; RMSE: 0.6 cc) and excellent reproducibility (ICC>0.90) compared to manual tracing, which exhibited greater variability (MAE: 16.94 cc; RMSE: 26.19 cc; ICC<0.50). Despite these limitations, manual methods may still offer clinical value in settings without access to advanced imaging technology.

This study provides early pregnancy reference intervals for ASV and supports VOCAL as a reliable and precise method for volume assessment. Manual techniques, while less consistent, remain feasible in low-resource contexts. These findings may contribute to more accurate gestational age estimation and early detection of developmental abnormalities.