With a normal NT and a critical risk for Down’s screening, non-invasive DNA testing is generally recommended in this case. Both NT test and Down’s screening are important screening tests for women during pregnancy and their accuracy is relatively high. If the NT test at 11-13 weeks plus 6 weeks of pregnancy is normal, and the Down’s syndrome test is critical risk, it is relatively unlikely that the fetus has a chromosomal abnormality. However, in the case of critical risk of Down’s syndrome, it is generally recommended to do further screening tests, which can be non-invasive DNA test, that is, by taking the blood of the pregnant woman, and extracting fetal DNA from the serum of the pregnant woman to determine whether the fetus has chromosomal abnormalities. Relatively speaking, the accuracy of non-invasive DNA is higher than that of Down’s syndrome screening. Therefore, if Down’s syndrome screening shows high risk or critical risk, non-invasive DNA testing is recommended.