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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 -
Prefrontal space ratio: comparison between trisomy 21 and euploid fetuses in the second trimester.
Sonek J, Molina F, Hiett AK, Glover M, McKenna D, Nicolaides KH.
Ultrasound Obstet Gynecol 2012;40:293-6.