How many times do you need to do the Down syndrome screening?

Patient's question:

Some say it's only done once, between 160,000 and 200,000. How come some say it needs to be done twice: one for the Down syndrome screening and another for the two-item Down syndrome screening... Please advise, thank you!

Doctor's answer:

Disease Analysis: Down syndrome screening only needs to be done once.
Guidance: The best time for Down syndrome screening is between 15-20 weeks of pregnancy. If a high-risk result is indicated, amniocentesis and fetal chromosome testing are also required.