Down syndrome (trisomy 21) is a disorder caused by the presence of an extra 21st chromosome.
唐氏综合症（21三体综合症）是由于患者额外多了一条21号染色体所致的疾病。
CONCLUSIONS: The extent to which free fetal DNA testing can be applied as a universal screening tool for trisomy 21, 18, and 13 depends mainly on assay accuracy and cost.
结论：胎儿游离dna检测作为21三体、18三体和13三体的通用筛查工具的程度，主要取决于测定方法的准确性和成本。
The detection rate of the chromosomal abnormality of the abnormal fetus detected by ultrasound (33.33%) was higer than the trisomy 21 high risk group(4.54%) and abnormal delivery group(9.09%)(P<0.05).
其中超声示胎儿异常组染色体异常检出率（33.33%）明显高于21-三体高风险组（4.54%）、不良孕产史组的检出率（9.09%）（P<0.05）。
Materials and Methods: Cerebral ct findings in 23 cases with 21 trisomy syndrome, proved by cytogenetics, were retrospectively analyzed.
材料与方法：回顾性分析23例经细胞遗传学检查证实的21三体综合征的脑部ct表现。
Conclusion: The ultrasonographic measurement data of nt have an important reference value in prenatal screening of fetal 21-trisomy syndrome.
结论：超声测量胎儿nt值，对2 1 -三体胎儿的筛查有重要的参考价值。
Conclusion: The ultrasonographic measurement data of nt have an important reference value in prenatal screening of fetal 21-trisomy syndrome.
结论：超声测量胎儿nt值，对2 1 -三体胎儿的筛查有重要的参考价值。
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