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References

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    Kagan KO, Wright D, Spencer K, Molina FS, Nicolaides KH. First-trimester screening for trisomy 21 by free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A: impact of maternal and pregnancy characteristics. Ultrasound Obstet Gynecol 2008; 31: 493502.
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    Pandya P, Wright D, Syngelaki A, Akolekar R, Nicolaides KH. Maternal serum placental growth factor in prospective screening for aneuploidies at 8–13 weeks' gestation. Fetal Diagn Ther 2012; 31: 8793.
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    Ball S, Wright D, Sodre D, Lachmann R, Nicolaides KH. Temporal effect of Afro Caribbean race on serum PAPP-A at 9–13 weeks' gestation in screening for aneuploidies. Fetal Diagn Ther 2012; 31: 162169.
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    Robinson HP, Fleming JE: A critical evaluation of sonar crown rump length measurements. Br J Obstet Gynaecol 1975; 182: 702710.
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    Wright D, Spencer K, Kagan KK, Tørring N, Petersen OB, Christou A, Kallikas J, Nicolaides KH. First-trimester combined screening for trisomy 21 at 7–14 weeks' gestation. Ultrasound Obstet Gynecol 2010; 36: 40411.
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    R Development Core Team. R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http://www.R-project.org/. 2011.
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    Wright D, Abele, H, Baker A, Kagan KO. Impact of bias in serum-free beta-human chorionic gonadotropin and pregnancy-associated plasma protein-A multiples of the median levels on first-trimester screening for trisomy 21. Ultrasound Obstet Gynecol 2011; 38: 309313.
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    Spencer K. The influence of smoking on maternal serum AFP and free beta hCG levels. Prenat Diagn 1998; 18: 225234.