How many weeks is the best time for Down syndrome screening? Down syndrome screening is a non-invasive screening test performed on pregnant women to determine whether the fetus is at risk for Down syndrome. As for the optimal time for Down syndrome screening, different literatures also have certain differences in their introduction. Next, Down syndrome screening will be introduced from four aspects: gestational age, examination methods, examination significance and related indicators. 1. The best time for gestational Down syndrome screening is between 16 weeks and 20 weeks of pregnancy. During this time period, the probability of fetal Down syndrome is highest, and pregnant women can be given sufficient time to develop.