Crown-rump length/nuchal translucency ratio as a supplemental screening marker in predicting birth defects: a two-phase retrospective study

Different cutoff values should be used in early pregnancy to determine whether nuchal translucency (NT) is abnormal. This study aimed to identify the optimal index for increased NT and explore a new method for standardizing NT and CRL as a supplemental marker for predicting birth defects. In phase 1, 575,674 pregnant women who underwent free measurement of NT were enrolled, and cutoff values for indices, such as CRL/NT, CRL/NT 2 , and NT/CRL 2 , were calculated. In Phase 2, 3,825 pregnant women, including group A, B and C, were recruited to compare the screening performance of traditional standards of increased NT and the new methods. Receiver operating characteristic (ROC) curve analysis showed that CRL/NT demonstrated comparable performance for all chromosomal abnormalities (AUC = 0.809, 95%CI :0.787–0.832), compared with CRL/NT 2 (AUC = 0.809, 95%CI : 0.786–0.831), NT/CRL 2 (AUC = 0.803, 95%CI :0.780–0.826), NT (AUC = 0.806, 95%CI :0.783–0.829). CRL/NT ≤ P 5 detected an additional 20.9% of chromosomal or structural abnormalities compared to NT ≥ P 95 . In conclusion, CRL/NT ≤ P 5 demonstrated screening performance comparable to NT≥ P 95 , while potentially identifying a small number of additional abnormal fetuses with a modest reduction in specificity. Therefore, it may warrant further evaluation as a supplemental screening indicator.