OP01.18: Combining fetal nuchal fold thickness with second‐trimester biochemistry to screen for trisomy 21

Abstract

To assess second-trimester screening for trisomy 21 by combining fetal nuchal fold (NF) with maternal serum biochemistry. NF and maternal serum α-fetoprotein (AFP) and free ?-hCG were determined on the same day, at 14–19 weeks of pregnancy. The study population comprised 1811 pregnancies, including 1256 unselected pregnancies undergoing routine risk assessment for trisomy 21 (1999–2002), and 555 high-risk pregnancies prior to amniocentesis (2003–5), 404 of which had positive serum screening tests. Results were expressed in multiples of the gestation-specific median (MoMs). There were 1799 unaffected singleton pregnancies, and their NF distribution approximated to Gaussian over a wide range of values with 5th-95th centile range of 0.63–1.47 MoM. There was a small but statistically significant correlation between NF and log AFP (R = − 0.081, P < 0.001) and an even smaller correlation between NF and free ?-hCG which was not significant (R = 0.035, P = 0.15). Among the 8 trisomy 21 pregnancies, the median NF level was 1.45 MoM (mean 1.74 MoM), a highly statistically significant increase (P < 0.001, one-tail Wilcoxon Rank Sum Test). In pregnancies referred because of a positive serum screening test (389 singleton unaffected, 2 twins, 7 trisomies 21, one monosomy X and 5 other chromosomal anomalies) the use of NF to modify the serum screening risk would reduce the invasive procedures by about a half without a change in the trisomy 21 detection rate. The addition of NF measurement to second-trimester biochemical screening improves screening performance, and could overcome the drawbacks in the implementation of Inhibin-A assay in clinical practice.