Multicenter Implementation and Validation of the Structured 3 × 3 CASSEAL Framework for Second‐Trimester Assessment of the Extended Fetal Cardiovascular System

Objectives To evaluate the feasibility, transferability, and implementation performance of the cardiovascular system sonographic evaluation algorithm (CASSEAL), a structured 3 × 3 sequential framework for second‐trimester ultrasound assessment of the extended fetal cardiovascular system, in a multicenter screening setting. Methods This prospective multicenter observational study included unselected singleton and twin pregnancies examined between 18 and 22 +6 weeks' gestation. After standardized training, operators completed an initial implementation phase (first 30 examinations) followed by a validation phase (≥50 additional examinations). The CASSEAL algorithm consists of sequential acquisition of 9 axial planes organized into infracardiac, cardiac, and supracardiac territories within a structured 3 × 3 framework. The primary outcome was complete visualization of all 9 views. Secondary outcomes included examination time, factors associated with incomplete visualization, and exploratory screening performance using absence of normality in ≥1 axial view as the screening criterion. Results A total of 1494 fetal examinations performed by 15 operators across 7 centers were analyzed. Complete visualization of all 9 views increased from 78% in the implementation phase to 84% in the validation phase ( p = .01). Mean examination time decreased significantly after completion of the learning phase (8.0 ± 6.4 versus 5.8 ± 3.9 minutes; p < .001). In multivariable analysis, the validation phase independently reduced the likelihood of incomplete visualization (adjusted OR, 0.65; p < .01), whereas higher maternal body mass index (adjusted OR, 1.06 per kg/m 2 ; p < .001) and anterior fetal spine position (adjusted OR, 3.95; p < .001) increased this risk. Thirty‐six fetuses (2.4%) had confirmed cardiovascular anomalies, vascular variants, or relevant ultrasound markers. Exploratory screening performance estimates based on available prenatal and postnatal follow‐up suggested high specificity and negative predictive value, although sensitivity estimates should be interpreted cautiously because systematic postnatal echocardiography was not available. Conclusions The CASSEAL algorithm demonstrates high feasibility, a short learning curve, and robust multicenter reproducibility for structured second‐trimester evaluation of the extended fetal cardiovascular system. The integrated 3 × 3 sequential framework may facilitate standardized assessment of supracardiac, cardiac, and infracardiac territories in routine multicenter screening practice.