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Screening for trisomy 21 by fetal tricuspid regurgitation, nuchal translucency and maternal serum free beta-hCG and PAPP-A at 11 + 0 to 13 + 6 weeks.
Falcon O, Auer M, Gerovassili A, Spencer K, Nicolaides KH.
Ultrasound Obstet Gynecol 2006;27:151-5 pdf