A screening program for trisomy 21 at 10-14 weeks using fetal nuchal translucency, maternal serum free β-human chorionic gonadotropin and pregnancy-associated plasma protein-A. - Spencer,K.; Souter,V.; Tul,N.; Snijders,R.; Nicolaides,K.H. (Ultrasound Obstet Gynecol, 1999)