The critical risk of trisomy 21 does not matter, the Down’s screening itself has a detection rate of 60-80% with some false positive rate. Pregnant women whose Down’s syndrome screening index is above normal should undergo chorionic gonadotropin testing or amniocentesis. If the test shows a constant, the occurrence of Down syndrome can be ruled out to the maximum extent possible. Down’s syndrome screening is age-restricted and is only indicated for pregnant women under 35 years of age. The advantage of Down’s syndrome screening is that it is economical, easy and non-invasive. The disadvantage is that it can only determine how likely the fetus is to have Down’s syndrome, but not definitively whether the fetus has Down’s syndrome. Therefore, if the risk is critical, non-invasive DNA or amniocentesis can be performed to further clarify the diagnosis, and if there is no abnormality, there is no need to worry too much.