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2. Prenatal diagnostic procedures;Gardner;Oxford Monographs on Medical Genetics,1996
3. Biparietal diameter/femur length ratio, cephalic index, and femur length measurements: not reliable screening techniques for Down syndrome;Shah;Obstet Gynecol,1990
4. Births: final data for 1998;Ventura;Natl Vital Stat Rep,1998
5. Efficacy of screening for fetal Down syndrome in the United States from 1974 to 1997;Egan;Obstet Gynecol,2000