Thymic-thoracic ratio combined with cardiac axis as a rapid primary screening index for fetuses with conotruncal anomalies: based on sequential cross-sectional scanning

Conotruncal anomalies are complex and variable and difficult to diagnose. The aim of this study is to investigate the diagnostic value of thymic-thoracic ratio (TT ratio) and Cardiac Axis(CAx) in rapid screening of fetal conotruncal anomalies. A single-center retrospective case-control study was conducted on fetuses from January 2021 to December 2024. Fetal CAx and TT ratio were routinely measured. The Mann–Whitney U test and the Kruskal–Wallis test were used to compare the data. Curve estimation and Pearson correlation were performed to assess the correlation between CAx and TT ratios and gestational age within the group. Logistic regression analysis and ROC curves were conducted. A total of 91 fetuses with conotruncal anomalies were identified and compared with 104 fetuses with non-conotruncal anomalies and 127 fetuses with normal structures. Compared with the normal group, the TT ratio was significantly reduced in the conotruncal anomalies group ( P < 0.001), and the CAx significantly skewed to the left ( P < 0.001). In the non-conotruncal anomalies group, only the CAx deviation was significantly different ( P < 0.001). Logistic regression analysis revealed that both TT ratio and CAx serve as indirect indicators of conotruncal anomalies, with the area under the ROC curve for the combined prediction model, including TT ratio and CAx, being 0.875. Measurement results demonstrated good consistency both between and within observer groups. CAx and TT ratio present distinct manifestations in fetuses with conotruncal anomalies. Their combined use yields potential diagnostic performance for rapidly identifying fetuses with conotruncal anomalies.