A model for second-trimester Down syndrome sonographic screening based on facial landmarks and digit length measurement

Objective: To determine whether Down syndrome can be detected by combining measurements of fetal nasal bone (NB) length, prenasal thickness (PT) and digits 2 and 3 of the hand. Methods: Two hundred and fifty-four normal and 25 Down-syndrome fetuses were scanned between 15 and 33 weeks' gestation. Physicians performing the scans were not blinded to the fetal karyotype. Both PT and NB were measured in a mid-sagittal plane. For PT measurement calipers were placed between the frontonasal angle and the outer skin edge. Digits 2 and 3 of one hand were also measured. The results (except for PT/NB ratio) were expressed in multiples of the normal gestation-specific median (MoM). A logistic regression model was used to estimate the odds of the fetus having Down syndrome given different combinations of NB, PT, PT/NB ratio, and digits 2 and 3 measurements. The odds were used to calculate the risk of Down syndrome for each pregnant woman from her age and measurements. Results: The median PT MoM for unaffected fetuses and Down-syndrome fetuses was 1.12 vs. 1.35 (T < 0.0001). The median NB MoM for unaffected and Down-syndrome fetuses was 1.03 vs. 0.81 (T < 0.001) and the PT/NB ratio MoM for unaffected and Down-syndrome fetuses was 0.63 vs. 0.96 (T < 0.001). The respective median MoM values for digits 2 and 3 of the Down-syndrome fetuses were significantly smaller (0.81 vs. 0.93 and 0.89 vs. 0.95, respectively, P = 0.003). Only the PT/NB ratio and digit 2 were finally included in the logistic regression equation. Using a 1 in 200 risk cut-off, the observed sensitivity and false-positive rate were 76% and 6.7%, respectively. Conclusion: Combining the PT/NB ratio and digit 2 measurements yielded a promising screening detection rate. Confirmation of our findings in a prospective study is needed before the method can be used clinically.