Conclusion
Adding PlGF or replacing PAPP-A with PlGF in the current first trimester combined test for trisomy 21 screening in screening centres performing both trisomy 21 and preeclampsia screening would not significantly improve the detection rate for trisomy 21 but would increase the screen positive rate if PAPP-A is replaced by PlGF or result in a small reduction in the screen positive and false positive rates if PlGF is added to the combined test.