OBJECTIVE: To establish reference intervals for the fetal cisterna magna volume by means of two-dimensional (2D) method using the multiplanar mode of three-dimensional ultrasonography. MATERIALS AND METHODS: Cross-sectional study with 224 healthy pregnant women between the 17th and 29th gestational weeks. The volume was automatically obtained by multiplying the three major axes in axial and sagittal planes by the constant 0.52. Polynomial regression was utilized to establish the correlation between fetal cisterna magna volume and gestational age, with adjustments by coefficient of determination (R²). Reliability and agreement were obtained by intraclass correlation coefficient (ICC) and limits of agreement, respectively. RESULTS: Mean fetal cisterna volume with the 2D method ranged from 0.71 ± 0.19 cm³ to 4.18 ± 0.75 cm³ at the 17th and 29th weeks, respectively. The authors observed a good correlation between fetal cisterna magna volume and gestational age (R² = 0.67), excellent intraobserver reliability and agreement with ICC = 0.89 and limits of agreement 95% (–52.0; 51.8), respectively, and low interobserver reliability and agreement with ICC = 0.64 and limits of agreement 95% (–110.1; 84.6), respectively. CONCLUSION: Reference intervals for fetal cisterna magna volume by means of 2D method using the multiplanar mode of three-dimensional ultrasonography were established and presented excellent intraobserver reliability and agreement.

Keywords: Fetus; Cisterna magna; Reference intervals; Two-dimensional ultrasonography; Three-dimensional ultrasonography.