Prefrontal space ratio: comparison between trisomy 21 and euploid fetuses in the second trimester.

OBJECTIVE To evaluate a novel ultrasound measurement, the prefrontal space ratio (PFSR), in second-trimester trisomy 21 and euploid fetuses. METHODS Stored three-dimensional volumes of fetal profiles from 26 trisomy 21 fetuses and 90 euploid fetuses at 15-25 weeks' gestation were examined. A line was drawn between the leading edge of the mandible and the maxilla (MM line) and extended in front of the forehead. The ratio of the distance between the leading edge of the skull and that of the skin (d(1)) to the distance between the skin and the point where the MM line was intercepted (d(2)) was calculated (d(2)/d(1)). The distributions of PFSR in trisomy 21 and euploid fetuses were compared, and the relationship with gestational age in each group was evaluated by Spearman's rank correlation coefficient (r(s) ). RESULTS The PFSR in trisomy 21 fetuses (mean, 0.36; range, 0-0.81) was significantly lower than in euploid fetuses (mean, 1.48; range, 0.85-2.95; P < 0.001 (Mann-Whitney U-test)). There was no significant association between PFSR and gestational age in either trisomy 21 (r(s) = 0.25; 95% CI, - 0.15 to 0.58) or euploid (r(s) = 0.06; 95% CI, - 0.15 to 0.27) fetuses. CONCLUSION The PFSR appears to be a highly sensitive and specific marker of trisomy 21 in the second trimester of pregnancy.