When Down syndrome screening is at a critical risk, a non-invasive DNA test is needed to definitively determine if Down syndrome is present in the fetus. When screening for Down syndrome, critical risk means that it is between high risk and low risk, which cannot clearly determine whether the fetus has Down syndrome or not, therefore, in order to ensure the health of the fetus, the next step of the test, that is, non-invasive DNA test, needs to be conducted to determine whether the fetus has Down syndrome or not. When the test result of a pregnant woman is a critical risk, she needs to consult a doctor in time, and under the doctor’s guidance, according to the individual’s situation, to carry out the next step of examination and treatment, to promote eugenics and the health of the baby.