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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 -
Ear length in trisomy 21 fetuses at 11-14 weeks of gestation.
Sacchini C, El-Sheikhah A, Cicero S, Rembouskos G, Nicolaides KH.
Ultrasound Obstet Gynecol 2003;22:460-3.