3Cuckle HS,Jan MML.Appropriate biochemical parameters in first-trimester screening for Down syndrome.Prenat Diagn 1999,19:505-512
4Spencer K,Berry E,Crossley JA,et al.Is matemal serum total hCG a marker of trisomy 21 in the first trimester of pregnancey?Prenat Diagn 2000,20:311-317
5Haddow JE,Palomaki GE,Knight GJ,et al.Screening of maternal serum for fetal DS in the first trimester.N Engl J Med 1998,338:955-961
6Pradya PP,Snijders RJM,Johnson SJ,et al.Screening for fetal trisomies by matermal age and fetal nuchal translucency thickness at 10-14 weeks of gestation Br J Obstet Gynaecol 1995,102:957-962
7Berger A,Science commentary :what is fetal nuchal translucency?Br Med J 1999,318:85
8Snijders RJM,Noble PL.Sebire NJ,et al.UK multicenter project on assaessment of risk of tigomy 21 by maternal age and muchal translucency thickness at 10-14 weeks of gestation.Lancet 1998,352:343-346
9Nicolaides KH,Azar G,Byme D,et al.Fetal nuchat translucency :ultrasound screening for chromosomal defects in first trimester of pregnancy.BMJ 1992,304:867-869
10Hafner E,Schuchter K,Liebhart E,et al.Results of routine fetal nuchal translucency measurement at weeks 10-13 in 4233 unselected pregnant women.Prenat Diagn 1998,18:29-34