Genetic sonography after first‐trimester Down syndrome screening

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Abstract
Objective Approximately 90% of Down syndrome cases are detected during first‐trimester screening. We aimed to determine the potential effectiveness of second‐trimester genetic sonography as a sequential screen for Down syndrome. Methods In this simulation study, published statistical parameters for first‐trimester free β‐human chorionic gonadotropin, pregnancy‐associated plasma protein‐A and nuchal translucency thickness, and second‐trimester ultrasound markers (nuchal fold, hyperechoic bowel, short humerus, short femur, echogenic intracardiac focus, pyelectasis and major abnormality) were used to model the effectiveness of second‐trimester genetic sonography combined with first‐trimester screening. Results First‐trimester combined screening alone resulted in a detection rate of 88.5% with a 4.2% false‐positive rate. A follow‐up genetic ultrasound examination in which only one sonographic marker was found and previous results were not taken into account would detect an additional 8% of Down syndrome cases for an additional false‐positive rate of 13.2%. Using individual marker likelihood ratios to modify the first‐trimester risk for screen‐negative patients, genetic sonography detected an additional 6.1% of Down syndrome cases for an additional 1.2% false‐positive rate, giving a total detection rate of 94.6% and a total false‐positive rate of 5.4%. In a contingent protocol, in which genetic sonography would be performed only for patients with a first‐trimester risk of between 1/300 and 1/2500, the detection rate was 4.8% and the false‐positive rate was 0.7%, giving a total detection rate of 93.3% and a total false‐positive rate of 4.9%. Conclusion Second‐trimester genetic sonography, if used properly, can be an effective sequential screen following first‐trimester Down syndrome screening. Further studies on the role of the genetic sonogram as a follow‐up to first‐trimester combined screening are warranted. Copyright © 2007 ISUOG. Published by John Wiley & Sons, Ltd.