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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 -
Maternal serum inhibin-A and free beta-hCG concentrations in trisomy 21 pregnancies at 10-14 weeks of gestation.
Noble PL, Wallace EM, Snijders RJ, Groome NP, Nicolaides KH.
BJOG 1997;104:367-71.