UK multicentre project on assessment of risk of trisomy 21 by maternal age and fetal nuchal translucency thickness at 10 - 14 weeks of gestation. Fetale medicine Foundation First Trimester Screening Group - Snijders,R.J.; Noble,P.; Sebire,N.J.; Souka,A.P.; Ncolaides,K.H. (Lancet, 1998)