Current screening strategies for Down syndrome, caused by fetal trisomy 21, and Edwards syndrome, caused by fetal trisomy 18, have false positive rates of 2-3 percent, and false negative rates of 5 percent or higher. Positive screening results must be confirmed by amniocentesis or CVS, carrying a fetal loss rate of approximately 1 in 300 procedures. Now an international, multicenter cohort study finds that a genetic test to screen for trisomy 21 or 18 from a maternal blood sample is almost 100 percent accurate.