Trisomy 21, also known as Down syndrome, refers to an extra chromosome 21, so that there are three chromosomes 21, so it is called 21-trisomy syndrome, in clinical work screening 21-trisomy syndrome, mainly through the Down’s screening, with 16-20 weeks of pregnancy as the best screening time period, the screening results in a risk value, and will be compared with the standard value. 21- trisomy high risk often suggests that the fetus has a higher probability of having 21-trisomy syndrome, but can not be used as a basis for confirmation of the diagnosis, further testing is required. A high risk of trisomy tends to indicate that the fetus has a higher probability of having 21-trisomy syndrome, but it is not a basis for a definitive diagnosis and further testing is needed.