Objective       To investigate the clinical significances of fetal ultrasound examination and prenatal diagnosis of fetal chromosome abnormality for critical-risk pregnancies of Down syndrome (DS) in second trimester. Methods       A retrospectively study was carried out on 2 281 pregnancies with critical-risk DS and on 2 500 low-risk cases admitted in our center from January 1

2012 to March 15

2016. Fetal ultrasonography was performed on all the 4 781 cases for the detection rates of single ultrasonographic soft markers

multiple ultrasonographic soft markers

single malformations

multiple malformations and other abnormalities. The results were compared between the 2 groups. Among those with critical-risk DS

1 332 cases who were aware of the situation accepted choose amniocentesis for prenatal diagnosis of fetal chromosomal abnormalities. Results        ①Fetal ultrasound examination detected 696 cases (30.51%) of single ultrasonographic soft markers

118 cases (5.17%) of multiple ultrasonographic soft markers

18 cases (0.79%) of single malformation

and 7 cases (0.31%) multiple malformations in the critical-risk group. But for the low-risk group

the above indicators were 308 cases (12.32%)

82 cases (3.28%)

5 cases (0.20%)

and 1 case (0.04%). The abnormal detection rates were significantly higher in the former than the latter group (P0.05). ②Amniocentesis indicated that among the 1 332 cases from critical-risk group

there were 135 cases complicated with single ultrasonographic soft markers

83 cases with multiple ultrasonographic soft markers

and 11 cases with single malformation. The 5 cases of multiple malformations had normal fetal chromosomal results.  ③The abnormal rate of chromosome was 0.08% (11 cases)

including 10 cases of autosomal and 1 of sex chromosomealstructural abnormalities. Among them

7 cases were chromosome balanced translocation

3 cases inversion

and 1 case arm chromosome. Nine cases of maternal or paternal origins

and 2 cases of de novo mutations. Conclusion       Fetal ultrasound examination can provide valuable information of genetic counseling for critical-risk DS pregnancies. Critical-risk DS also indicates increased risk of fetal chromosomal structure abnormalities.