First‐trimester screening for trisomy 21 combining biochemistry and ultrasound at individually optimal gestational ages. An interventional study

Abstract
Objective: To assess the effectiveness of the Combined Test in the prenatal detection of trisomy 21 in the general pregnant population using a new timing for the screening approach.Methods: First‐trimester maternal serum biochemical markers (pregnancy‐associated plasma protein‐A and free‐β hCG) were determined in maternal serum at 7 to 12 weeks. Fetal nuchal translucency and gestational age were assessed at the 10‐ to 14‐week ultrasound scan. A combined risk was estimated and delivered to the women the same day. When the risk was 1:250 or above, chorionic villus sampling was offered.Results: Mean gestational age at biochemistry was 9.4 weeks, being 12.3 at ultrasound. In the 2780 studied pregnancies with a complete follow‐up, observed detection rates were 88% (7/8) for trisomy 21 and 75% (3/4) for trisomy 18, with a 3.3% (92/2765) false‐positive rate.Conclusion: The Combined Test, assessing biochemistry and ultrasound at individually optimal ages in the first trimester, showed an 88% detection rate for trisomy 21 with a remarkably reduced false‐positive rate (3.3%). Copyright © 2004 John Wiley & Sons, Ltd.

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