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Screening at 11-13+6 weeks' gestation.
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A screening program for trisomy 21 at 10-14 weeks using fetal nuchal translucency, maternal serum free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A.
Spencer K, Souter V, Tul N, Snijders R, Nicolaides KH.
Ultrasound Obstet Gynecol 1999;13:231-7.