The risk of trisomy 21 is generally referred to as trisomy 21 screening, also known as the critical risk of Down’s syndrome screening, which is determined by a risk value of more than 1/270 but less than 1/1000 for Down’s syndrome screening, which means that although the fetus is not at high risk of developing trisomy 21, the prevalence is still relatively high. The screening for Down’s syndrome is usually performed between 12 and 22 weeks of gestation, and the risk of having a fetus with congenital defects including trisomy 21, trisomy 18 and neural tube abnormalities is calculated by taking the serum of the pregnant woman and testing the concentration of the relevant indicators in the maternal serum, and combining the expected date of delivery, weight, age and the week of gestation when the blood is taken. The normal value should be less than 1/270, indicating a low risk of having a child with trisomy 21. If it is greater than this value, it is a positive screening test and needs to be clarified by non-invasive DNA testing or amniocentesis depending on the maternal age. If the age is below 35 years, a non-invasive fetal DNA test can be considered to predict the risk of fetus with trisomy 21 by collecting maternal peripheral blood, sequencing free DNA fragments including free DNA in maternal peripheral plasma, and performing bioinformatics analysis, but for those who detect high risk results, further definitive diagnosis is needed and the decision to The clinical decision to terminate a pregnancy should not be based solely on the results of noninvasive DNA testing. If the fetus is over 35 years of age and is willing to take the risk of amniocentesis, laparotomy may be considered to extract amniotic fluid for karyotyping of fetal cells to confirm the diagnosis of trisomy 21, which is usually associated with fetal heart malformations, cleft lip and palate, and other malformations. Once trisomy 21 is diagnosed, the pregnancy usually needs to be terminated.