The genetic probability of trisomy 21 does not usually need to be calculated, there is no specific formula for calculating it, it is mainly measured by hospital instruments, the critical value is about 1:270, if it exceeds the critical value, then it indicates that there is a risk of the disease. Trisomy 21 refers to Down’s syndrome, a congenital fetal abnormality caused by chromosomal abnormalities, which may be related to the age and weight of the pregnant woman as well as the week of pregnancy. The probability of inheritance of this disease cannot be calculated, and pregnant women can go to the hospital to determine it through Down’s syndrome screening, non-invasive DNA testing and amniocentesis. Generally speaking, the threshold for Down’s syndrome is about 1:270. If the result of the test is less than 1:270, then it means that the risk is low and the fetus has a low chance of developing trisomy 21. However, if the risk of trisomy 21 is greater than the threshold, then the risk is high, and further testing through non-invasive DNA or amniocentesis is needed to confirm the diagnosis. Since there is no effective treatment for this disease, it is important to terminate the pregnancy as soon as possible under the supervision of a doctor to avoid any impact on the health of the fetus.