Second-trimester screening for trisomy-21 using prefrontal space ratio.

OBJECTIVE To investigate the potential value of prefrontal space ratio (PFSR) in second-trimester screening for trisomy-21. METHODS A retrospective study utilizing stored midsagittal two-dimensional images of fetal profiles in 240 euploid and 45 trisomy-21 pregnancies at 16(+0)-23(+6) weeks' gestation. The vertical distance between the leading edge of the skull and that of the skin (D1) and the distance between the skull and the mandibulo-maxillary line (D2) were measured and the D1:D2 ratio (PFSR) was calculated. In euploid pregnancies, regression analysis was used to determine the association between D1, D2 and PFSR with gestational age (GA). D1 and D2 were expressed as delta (Δ) values with gestational age. ΔD1, ΔD2 and PFSR in cases and controls were compared. RESULTS In trisomy-21, compared to controls, ΔD1 was increased (1.417 vs. 0.000 mm, p < 0.0001), ΔD2 was decreased (-0.842 vs. 0.000 mm, p = 0.003) and PFSR was increased (0.753 vs. 0.463, p < 0.0001). At a false-positive rate of 5%, the detection rates in screening by ΔD1, ΔD2 and PSFR were 80.0% (95% CI 65.4-90.4), 46.7% (95% CI 31.7-62.1) and 100.0% (95% CI 92.1-100.0), respectively. CONCLUSION The PFSR is an effective marker in second-trimester screening for trisomy-21.