What is the difference between four-dimensional color Doppler ultrasound and Down's screening?

Down's screening is to take pregnant women's blood in the middle of pregnancy, according to the blood hcg, alpha-fetoprotein, pregnant women's weight and age to comprehensively assess risk factors. Four-dimensional color Doppler ultrasound examination is based on ultrasound to check whether there are any organ development abnormalities. Four-dimensional color Doppler ultrasound can be used to check whether there are obvious development abnormalities in various organs of the fetus. Down's screening is mainly aimed at congenital Down's screening. If four-dimensional color Doppler ultrasound indicates a developmental abnormality, induction may be necessary. If Down's screen shows a high risk, amniocentesis may be required.