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First-trimester contingent screening for trisomy 21 by biomarkers and maternal blood cell-free DNA testing.
Nicolaides KH, Wright D, Poon LC, Syngelaki A, Gil MM.
Ultrasound Obstet Gynecol 2013;42:41-50. pdf -
Integrated ultrasound and biochemical screening for trisomy 21 using fetal nuchal translucency, absent fetal nasal bone, free beta-hCG and PAPP-A at 11 to 14 weeks.
Cicero S, Bindra R, Rembouskos G, Spencer K, Nicolaides KH.
Prenat Diagn 2003;23:306-10.