What is the basis for checking the risk rate of Down syndrome?

Patient's question:

Hello doctor Tang Shi screening, the risk rate of Down syndrome is 1:350 (1:270 low risk ≥1:270 high risk). Are these data normal? Thank you

Doctor's answer:

Analysis of the condition: The triple screen test is based on the concentration of alpha-fetoprotein and human chorionic gonadotropin in the pregnant woman's serum, combined with the pregnant woman's age, gestational age, weight, and other factors, using a computer to precisely calculate the probability of each pregnant woman developing Down syndrome.
Advice: A triple screen risk level below 1/270 is normal. Your result is 1/350, which is within the normal range, indicating that the probability of the fetus being affected is less than 1%. There is no need to worry, and regular prenatal examinations can be performed.