Screening for trisomy 21 by maternal age, fetal nuchal translucency thickness, free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A - Kagan, K O; Wright, D; Baker, A; Sahota, D; Nicolaides, K H (Ultrasound Obstet Gynecol, 2008)
. Doi: 10.1002/uog.5331