Trisomy 21 values include amniocentesis chromosome, Down’s syndrome screening test results and non-invasive DNA test results, as follows: 1. Down’s syndrome screening: Trisomy 21 or Trisomy 18 is lower than 1:270, which is a low risk group and no need to continue the test. If the risk value of trisomy 21 or trisomy 18 is 1:270-1:1000, it is a critical high risk, which requires further examination, non-invasive DNA or amniocentesis, because the accuracy of Down’s screening is not high; 2. non-invasive DNA: low risk of trisomy 21, i.e. the test value is less than 2.5, which indicates no abnormal fetal growth and development; if there is a high risk, i.e. the test value is greater than 3, it is necessary to If the test value is higher than 3, further amniocentesis is required; 2.6-3 is the critical risk, and amniocentesis is also recommended. Non-invasive DNA is more accurate, so if there is a high risk of trisomy 21, the fetus has a higher probability of Down’s syndrome; 3. Amniocentesis: that is, amniotic fluid is extracted by puncture in the amniotic cavity, amniotic fluid cell culture is performed, and fetal chromosome karyotype analysis is performed to determine whether there is a chromosomal abnormality. If the phospholipid to sphingomyelin ratio (L/S) ≥ 2, suggesting that the fetal lung has matured, L/S ≤ 1.9, indicating that the fetal lung has not matured, the newborn is prone to respiratory distress syndrome; AFP (AFP) normal value < 25μg/L. If 10 times higher than normal, there will be open neural tube malformation; amniotic fluid estriol (E3) normal value ≥ 100μg/ml, if < 100μg/ There is no effective treatment for trisomy 21, and the best solution is to terminate the pregnancy before delivery. However, it can be prevented through prenatal prevention, such as genetic counseling and prenatal diagnosis.