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First-trimester contingent screening for trisomies 21, 18 and 13 by fetal nuchal translucency and ductus venosus flow and maternal blood cell-free DNA testing.
Kagan KO, Wright D, Nicolaides KH.
Ultrasound Obstet Gynecol 2015;45:42-7. pdf -
Maternal serum free beta-hCG at 10 to 14 weeks of gestation in trisomic twin pregnancies.
Noble PL, Snijders RJ, Abraha HD, Sherwood RA, Nicolaides KH.
BJOG 1997;104:741-3.