Abstract: Objective To detect the prenasal thickness(PT) of fetuses with prenatal ultrasound test,investigate the predictive role of prenasal thickness for fetuses with Down syndrome,and determine the multiples of the median(MoM) value of PT in ultrasonic examination,and assess the clinical value of PT in screening for Down syndrome fetuses. Mesthods PT was measured in normal and DS fetal at 16-25 weeks of gestation in our hospital from June,2012 to June,2014.The Student’s t-test or Wilcoxon rank sum test was used to compare the mean difference between the two groups.Spearman’s Rank-order Correlation was implemented to determine the correlation of two variables.The MoM(PT) value of two groups was calculated by using a quadratic model for the Trisomy risk assessment.The sensitivity,specificity,positive and negative predictive value,positive and negative likelihood ratio were observed. Results The average PT in the normal group was (3.51±0.91)mm,and in the DS group was (5.16±0.79)mm,the difference was statistically significant(t=12.23,P<0.05) PT increased with the gestational age.MoM(PT)values in DS and unaffected pregnancies were 1.371 and 1.010 MoM,respectively.Using PT alone for screening of DS,the false positive rate was 5% and the sensitivity was predicted to be 58%,the positive predictive value was 0.46,the negative predictive value was 0.97,the positive likelihood ratio was 11.63,the negative likelihood ratio was 0.44;When combining of PT with high risk age,the false positive rate was 5% and the sensitivity was increased to 75%,the positive predictive value was 0.52,the negative predictive value was 0.98,the positive likelihood ratio was 14.94,the negative likelihood ratio was 0.26. Conclusion PT is considered as the effective sonographic marker to screen Down syndrome in the second trimester.PT of fetuses with Down syndrome is thicker than that of the unaffected pregnancies.The combination of PT and pregnant women age will obviously increase the detection rate of Down syndrome fetus.When the risk cut-off levels are > or =1 in 250 for Down syndrome,the invasive examination should be performed.