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Second-trimester screening for trisomy-21 using prefrontal space ratio.
Chaveeva P, Agathokleous M, Poon LC, Markova D, Nicolaides KH.
Fetal Diagn Ther 2013;34:50-5. pdf -
Maxillary length at 11-14 weeks of gestation in fetuses with trisomy 21.
Cicero S, Curcio P, Rembouskos G, Sonek J, Nicolaides KH.
Ultrasound Obstet Gynecol 2004;24:19-22.