Down's screening is to do B ultrasound?

Tang's screening is not to do B-ultrasound, mainly to extract pregnant women's serum for testing, and then combined with the pregnant woman's age, gestational age, weight, etc., to calculate the risk coefficient of congenital defects fetus, this is also a pregnancy must do an examination, if Tang's screening risk coefficient is high, but also to do a further amniocentesis examination.