What is Tang screening test? Down screening is a type of prenatal screening for pregnant women that can detect whether the fetus has chromosomal abnormalities such as Down syndrome. This examination is performed in the first trimester, including blood testing at 13-14 weeks of pregnancy and B-ultrasound examination at 18-22 weeks of pregnancy. It is widely used in clinical practice because of its safety, non-invasiveness and high accuracy. The most accurate time for pregnancy screening is to conduct a blood test at 13-14 weeks of pregnancy, and at 18-14 weeks of pregnancy.