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First-trimester screening for trisomies 21, 18 and 13 by ultrasound and biochemical testing.
Wright D, Syngelaki A, Bradbury I, Akolekar R, Nicolaides KH.
Fetal Diagn Ther 2014;35:118-26. pdf -
First-trimester maternal serum PAPP-A, SP1 and M-CSF levels in normal and trisomic twin pregnancies.
Bersinger NA, Noble P, Nicolaides KH.
Prenat Diagn 2003;23:157-62.