The risk value for trisomy 21 should normally be below the reference value, i.e., less than 1:270. If it is below this value, the child is considered to have a very low likelihood of developing trisomy 21. If it is significantly higher than this reference range, the child is considered to be at high or intermediate risk of developing trisomy 21, both of which are dangerous. Further non-invasive DNA testing or amniocentesis should be performed. However, the low risk of trisomy 21 is only a predictive value and cannot be used as a diagnostic criterion for the final diagnosis of the child’s trisomy 21 status, so it should be evaluated in conjunction with other clinical diagnostic indicators.