The normal value for Down’s syndrome screening is 1:270. The smaller the value, the lower the risk of developing trisomy 21, and the larger the ratio, the higher the risk of developing the disease. The normal value for Down’s syndrome screening varies depending on the method of screening. The Down’s syndrome screening is performed by taking the blood of a pregnant woman and separating the serum to detect the values of alpha-fetoprotein, human chorionic gonadotropin and free estriol in the serum, and assessing the risk of having a child with Down’s syndrome in combination with the age and week of pregnancy of the pregnant woman. Down syndrome screening is a preliminary test that is performed in the middle of pregnancy. If there is a risk of the disease, non-invasive DNA testing is required to further determine the presence of Down syndrome, a genetic disorder that cannot be cured by current medicine once the affected child is born.