The upper limit of the critical risk value for trisomy 21 is 1:270 and the lower limit is 1:1000, and further testing should be done when it is abnormally elevated. 1. Trisomy 21 is an important test in Down’s syndrome screening. Normally, the standard value of trisomy 21 is 1:270, which suggests that the fetus has a high probability of having trisomy 21 and a high likelihood of having a Down’s child, and further non-invasive DNA test or amniocentesis should be performed to make a definite diagnosis. 2. The lower limit of risk for trisomy 21 is 1:1000, below which the fetus is very unlikely to have the disease, but regular checkups are needed. 3. Between 1:270 and 1:1,000 is called the critical risk, which is low risk but closer to the standard value, so there is still a certain risk of trisomy 21, so further non-invasive DNA testing is needed. It is important to note that the accuracy of Down’s syndrome screening is not 100%, and when abnormalities occur in the fetus, medical attention should be sought as soon as possible to avoid delays that could lead to adverse pregnancy outcomes.