8Wright D, Bradbury I, Benn P, et al. Contingent screening for Down syndrome is an efficient alternative to non - disclosure se - quential screening [ J ]. Prenat Diagn,2004,24 ( 10 ) :726 - 766.
9Palacio M ,Jauniaux E, Kingdom J, et al. Perinatal outcom in pregnan- cies with a positive serum screening for Downg syndrome due to ele- vated levels of free - beta - human chorionic gonadotropin [ J ]. Uhra- sound Obster Gynecol, 1999,13 ( 1 ) :58 - 62.