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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 -
Fetal head-to-trunk volume ratio in chromosomally abnormal fetuses at 11 + 0 to 13 + 6 weeks of gestation.
Falcon O, Cavoretto P, Peralta CF, Csapo B, Nicolaides KH.
Ultrasound Obstet Gynecol 2005;26:755-60.