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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 AFP and total hCG in aneuploidies other than trisomy 21.
Spencer K, Heath V, Flack N, Ong C, Nicolaides KH.
Prenat Diagn 2000;20:635-9.