Performance of first trimester screening for Trisomy 21 in twin pregnancies

Objectives To assess the performance of the Danish first trimester screening program in twin pregnancies. Method Retrospective, nation-wide, cohort study with data collected from the Danish Fetal Medicine Database (DFMD) and The Danish Central Cytogenetic Registry (DCCR). The cohort included all women with twin pregnancies participating in the national first trimester screening program for Trisomy 21. Risk assessment was based on maternal age, nuchal translucency (NT) thickness and, if available, biochemical markers (beta-hCG and PAPP-A). Results: 8812 twin pregnancies including 42 pregnancies with Trisomy 21 had a risk assessment between 2009 and 2017. The detection rate (DR) for pregnancies with a risk assessment based on maternal age and NT only (missing data on biochemical markers, n = 4693) was 69.6% (95% CI: 50.8%-88.4%) for a 6.3% false positive rate (FPR) (95% CI: 5.6%-7.0%), whereas for pregnancies with a risk assessment based on all three parameters (n = 4119) the DR was 89.5% (95% CI: 76.7%-100.0%) for a 7.2% FPR (95% CI: 6.4%-8.0%). Conclusion The DR of Trisomy 21 in twin pregnancies, seems as high as for singleton pregnancies, when using optimal screening techniques, but the FPR is nearly twice as high.