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Maternal serum biochemistry at 11-13(+6) weeks in relation to the presence or absence of the fetal nasal bone on ultrasonography in chromosomally abnormal fetuses: an updated analysis of integrated ultrasound and biochemical screening.
Cicero S, Spencer K, Avgidou K, Faiola S, Nicolaides KH.
Prenat Diagn 2005;25:977-83.