1Brambati B, Macintosh MCH, Teisner B, et al. Low maternal serum levels of pregnancy associated plasma protein A (PAPP-A) in the first trimester in association with abnormal fetal karyotype. Br J Obstet Gynaecol, 1993,100:324-326.
2Wald NJ, George L, Smith D, et al. Serum screening for Down′s syndrome between 8 and 14 weeks of pregnancy. Br J Obstet Gynaecol, 1996, 103:407-412.
3Macintosh MC, Iles R, Teisner B, et al. Maternal serum human chorionic gonadotrophin and pregnancy-associated plasma protein A, markers for fetal Down syndrome at 8-14 weeks. Prenat Diagn, 1994, 14:203-208.
4Forest JC, Mass J, Moutquin JM, et al. Screening for Down syndrome during first trimester: a prospective study using free beta-human chorionic gonadotropin and pregnancy-associated plasma protein A. Clin Biochem, 1997, 30:333-338.
4Spencer K, Nicolaides KH. A first trimester trisomy 13 / trisomy 18 risk algorithm combining fetal nuchal translucency thickness, maternal serum free β-hCG and PAPP-A[J]. Prenat Diagn, 2002,22:877-879.
5Biagiotti R, Brizzi L, Periti E, et al. First trimester screening for Down's syndrome using maternal serum PAPP-A and freeβ-hCG, in combination with fetal nuchal translucency thickness[J]. Br J Obstet Gynaecol, 1998,105:917-920.
6Wald NJ, George L, Smith D, et al. Serum screening for Down's syndrome between 8 and 14 weeks of pregnancy[J]. Br J Obstet Gynaecol, 1996, 103:407-412.
7Casals E, Aibar C, Martinez JM, et, al. First-trimester biochemical markers for down syndrome[J]. Prenat Diagn, 1999,19:8-11.
8Spencer K, Tul N, Nicolaides KH. Maternal serum free β-hCG and PAPP-A in fetal sex chromosome defects in the first trimester[J]. Prenat Diagn, 2000, 20:390-394.
9Biasio PD, Siccardi M,Volpe G, et al. First-trimester screening for down syndrome using nuchal translucency measurement with freeβ-hCG and PAPP-A between 10 and 13 weeks of pregnancy-the combined test[J]. Prenat Diagn, 1999,19:360-363.
10Spencer K, Ong C, Skentou H, et al. Screening for trimester 13 by fetal nuchal translucency and maternal serum freeβ-hCG and PAPP-A at 10-14 weeks of gestation[J]. Prenat Diagn, 2000,20:411-416.