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, K H (Ultrasound Obstet Gynecol, 1999)
. Doi: 10.1046/j.1469-0705.1999.13040231.x