Frontomaxillary facial angle in screening for trisomy 21 at 11 + 0 to 13 + 6 weeks.

OBJECTIVE Trisomy 21 is associated with a flat face, which can now be quantified by measurement of the frontomaxillary facial (FMF) angle. The aim of this study was to examine whether in trisomy 21 fetuses fetal nuchal translucency (NT) thickness and maternal serum free ss-human chorionic gonadotropin (ss-hCG) and pregnancy-associated plasma protein-A (PAPP-A) are independent of the FMF angle, and to estimate the performance of a first-trimester screening test for trisomy 21 that includes measurement of the FMF angle. METHODS This was a prospective study in singleton pregnancies at 11 + 0 to 13 + 6 weeks of gestation in which three-dimensional volumes of the fetal head were obtained and measurement of the FMF angle performed immediately before fetal karyotyping by chorionic villus sampling (CVS). The women chose to have CVS after risk assessment by a combination of maternal age, fetal NT thickness and maternal serum free ss-hCG and PAPP-A. Regression analysis was used to examine the significance of the association within the euploid and within the trisomy 21 fetuses between the deviation from the normal median in FMF angle and the deviation in NT, free ss-hCG and PAPP-A. We estimated the detection rate (DR) and false positive rate (FPR) of first-trimester screening for trisomy 21 by measuring the FMF angle in all cases and of an alternative policy in which first-stage screening is by fetal NT and maternal serum biochemistry in all patients, followed by second-stage assessment of FMF angle only in those with an intermediate risk (1 in 51 to 1 in 1000) after the first stage. RESULTS The FMF angle was measured in 782 euploid and 108 trisomy 21 fetuses. In the euploid fetuses the mean FMF angle decreased linearly with CRL from 83.5 degrees at a crown-rump length (CRL) of 45 mm to 76.4 degrees at a CRL of 84 mm. In the euploid fetuses the mean delta FMF angle was 0.0 (SD, 4.264) degrees and the respective values in the trisomy 21 fetuses were 7.172 (SD, 4.092) degrees . Incorporating the FMF angle in first-trimester combined screening increased the estimated DR from 90 to 94% at an FPR of 5% and from 85 to 92% at an FPR of 3%. In two-stage screening it would be necessary to measure the FMF angle in 12% of cases and the DRs would be 93 and 91% at FPRs of 5 and 3%, respectively. CONCLUSIONS Measurement of the FMF angle improves the performance of first-trimester screening for trisomy 21.