Abstract / Summary
Background/Objectives: Fetal aortic isthmus reference data have characterized its size as a function of gestational age; whether aortic and pulmonary valve diameters are associated with isthmic caliber in addition to gestational age has not been systematically examined. We examined the correlates of aortic isthmus diameter in fetuses considered structurally normal on second-trimester assessment. Methods: This retrospective single-center cohort study included 472 fetuses considered structurally normal on second-trimester assessment at 19 + 0–24 + 6 weeks. Aortic isthmus (AoI), aortic valve (AV), pulmonary valve (PV) and thymic transverse diameters were measured by a single operator. Three-step hierarchical linear regression modeled the AoI diameter (Model 1, gestational age; Model 2, +AV and PV; Model 3, +thymus), with variance inflation factors, regression diagnostics and internal validation conducted via repeated five-fold cross-validation (100 repeats). For sensitivity analyses, the gestational-age equivalents recorded in the clinical reports for femur length, biparietal diameter and head circumference were converted into total days and analyzed as continuous variables in place of the clinical gestational age. Results: AoI diameter was correlated with gestational age (r = 0.512), AV (r = 0.507), PV (r = 0.516) and thymic diameter (r = 0.397) (all p < 0.001). Gestational age alone accounted for 26.2% of the variance (R2 = 0.262). Adding AV and PV increased the explained variance (ΔR2 = 0.060; total R2 = 0.322; p < 0.001); both AV (β = 0.186; p = 0.005) and PV (β = 0.187; p = 0.006) were independently associated with isthmic caliber, and the highest VIF in Model 2 was 3.22. Adding thymic diameter produced no meaningful increment (ΔR2 = 0.0003; p = 0.636). Across the four size-index specifications, the valve-term increment ranged from 0.037 to 0.060. Internal validation via repeated five-fold cross-validation gave a mean cross-validated R2 of 0.299 (SD of repeat-specific means, 0.011). Conclusions: In fetuses considered structurally normal on second-trimester assessment, aortic isthmus diameter was associated with gestational age, and, in addition, with aortic and pulmonary valve diameters, which accounted for a modest further share of the variance. No independent association with thymic diameter was demonstrated.