Gestational age-specific reference ranges and Z-score equations for fetal aortic and pulmonary valve annulus diameters at 18–24 weeks of gestation

Purpose To establish gestational age-specific reference ranges and internally validated Z -score equations for fetal aortic (AV) and pulmonary valve (PV) annulus diameters between 18 + 0 and 24 + 6 weeks of gestation in a routine screening population, and to evaluate the PV/AV ratio as a parameter of proportionate semilunar valve growth. Methods In this single-center retrospective cross-sectional study, 2,662 singleton fetuses were examined. AV and PV annulus diameters were measured in systole at maximal opening by a single operator. Linear regression models were fitted using gestational age in days, with gestational age-dependent standard deviations to account for increasing residual variance. Held-out internal validation was performed by tenfold cross-validation, with the complete modeling procedure refitted within each training fold and applied to untouched held-out folds; bootstrap resampling was used to assess coefficient stability. Results The final regression equations were AV = 2.9273 + 0.0342 × (GA − 150) ( R 2 = 0.370) and PV = 3.6650 + 0.0420 × (GA − 150) ( R 2 = 0.395). In held-out validation, the pooled mean Z -score was + 0.023 (SD 1.048) for AV and + 0.016 (SD 1.046) for PV, with 93.86 and 93.49% of observations within ± 1.96 Z -score units (nominal 0, 1 and 95%). The mean PV/AV ratio was 1.262 (SD 0.146) and remained stable across gestation ( R 2 < 0.001). Conclusions These internally validated reference ranges and Z -score equations may support standardized second-trimester assessment of fetal semilunar valve dimensions. The stable PV/AV ratio may provide an additional parameter for assessing proportionate semilunar valve growth during routine anomaly scanning.