The normal value of Trisomy 21 is around 1:700, but the reference value may vary from hospital to hospital. The Trisomy 21 test, also known as Down’s syndrome screening, is performed at 15-20 weeks of pregnancy to determine the risk of Down’s syndrome in the fetus by testing the pregnant woman’s blood and combining it with her age, week of pregnancy, and maternal history, etc. A test result between 1:250-1:380 is considered a critical risk, while a test result of less than 1:700 is called low risk, and a test result of more than 1:270 is called high-risk. . Since screening for trisomy 21 is a rough projection, low risk does not necessarily mean it is safe, and high risk does not necessarily mean it will develop the disease; further testing is needed in conjunction with the pregnant woman’s family history, genetic history, and so on.