Trisomy 21 critical risk is not universal and is between high and low risk, meaning that the fetus cannot be completely ruled out as having Down’s syndrome. The results of Down’s screening are categorized as low risk, critical risk and high risk, which is not very accurate and is subject to false positives and false negatives. Therefore, whenever the result of the test is critical risk or high risk, further non-invasive DNA or amniocentesis tests are required to further confirm whether the fetus has Down syndrome. Either of these two tests is about ninety-nine percent accurate. Although the chances of having a Down’s syndrome fetus with a critical risk of trisomy 21 are not great, further tests are needed to rule it out, and it is recommended to go to a regular hospital for a clear diagnosis in a timely manner.