Prenatal screening for Down syndrome in twin pregnancies: Estimates of screening performance based on 61 affected and 7302 unaffected twin pregnancies. (16th December 2018)

• Record Type: Journal Article
• Authors: Wald, Nicholas J.; Bestwick, Jonathan P.; Huttly, Wayne J.; Aldis, Jonathan; Borrell, Antoni; Goodburn, Sandy; Mills, Ian
• Is Part Of: Prenatal diagnosis. Volume 38:Number 13(2018)
• Page Start: 1079
• Page End: 1085
• Publication Date: 2018-12-16
• Subjects: Prenatal diagnosis -- Periodicals; Fetus -- Diseases -- Diagnosis -- Periodicals; Electronic journals; 618.32075
• DOI: 10.1002/pd.5381
• Languages: English
• ISSNs: 0197-3851

Abstract

Abstract: The aims of this study were to determine whether assumptions used in prenatal screening for Down syndrome in twin pregnancies are valid and derive estimates of risk and screening performance in twin pregnancies using observed data. Data were collected on nuchal translucency, chorionicity, pregnancy associated plasma protein‐A (PAPP‐A), and free ß human chorionic gonadotrophin (free ß‐hCG) from 61 twin pregnancies with Down syndrome and 7302 unaffected twin pregnancies. Distribution parameters were determined and used to estimate screening performance. The assumption that proportional differences in serum marker levels in affected and unaffected singleton pregnancies apply to twin pregnancies was not confirmed. Median free β‐hCG value in monochorionic affected twin pregnancies (2.63 multiples of the median [MoM]; 95% CI, 1.79‐3.22 MoM) was lower than that assuming proportionality (3.76 MoM), and the median PAPP‐A value in dichorionic affected twin pregnancies (1.88 MoM; 95% CI, 1.60‐2.17 MoM) was higher than that based on proportionality (1.33 MoM). The detection rate was 87% for a 3% false‐positive rate in monochorionic twin pregnancies and 74% in dichorionic twin pregnancies compared with 86% in singleton pregnancies. Estimates of screening performance in Down syndrome twin pregnancies do not need to rely on assumptions and can take account of chorionicity and gestational age. Abstract : What's already known about this topic? Prenatal Down syndrome screening markers in unaffected twin pregnancies are approximately double those in singleton pregnancies. It has generally been assumed that the proportional differences in serum marker levels in affected and unaffected singleton pregnancies can be applied to levels found in twin affected and unaffected pregnancies. What does this study add? The median late first‐trimester free β‐hCG level in monochorionic affected twin pregnancies is lower than levels based on the proportionality assumption, and the median PAPP‐A level in dichorionic affected twin pregnancies is higher. Combined test screening performance in monochorionic twins is similar to that in singletons, but lower in dichorionic twins. Different combined test risk cut‐offs in monochorionic and dichorionic twin pregnancies would be needed to achieve similar false‐positive rates.