In this multicenter study, we collected a total of 52 babies of 18-trisomy syndrome (34 cases were prenatally diagnosed and 18 failed to be diagnosed), and analyzed their characteristics. In the past 13 years, according to the data from the prenatal diagnosis center in Jiangsu, the estimated detection rate was 65.4% for trisomies 18 in the second trimester. After analyzed the distribution of the groups according to T18-risk, there were two important cut-off value to be noted: 1/455 and 1/5050. It was worthwhile noting that the risk of 65.4% trisomy 18 cases were higher than 1/455, 90.4% higher than 1/5050. However, 96.1% cases in normal control group were lower than 1/5050. No case from normal control group would be higher than the value of risk in 1/450. Meanwhile, the T18-risk of 50% T18 failed to be diagnosed cases were 1/1001~1/5050. If we chosen NIPT follow the standard of 1/5050, we could found 90.4 % T18 cases. It could increased 19.2% T18 babies be detected, but 3.2% normal mother would accepted the NIPT examination which maybe unnecessary. The prediction mode of HCG MoM and AFP MoM might be able to help us reduce the failed to be diagnosedis. It is also necessary to adjust more reasonable range of NIPT with further clinical researches.