How many months of pregnancy is the Down syndrome screening done?

Patient's question:

Perform Down syndrome screening

Doctor's answer:

Disease Analysis: Down syndrome screening should be performed between weeks 14-20 of pregnancy, with the optimal time being weeks 16-20. Testing beyond week 20 has no reference value.
Guidance: Between weeks 23-26 of pregnancy, a 3D or 4D ultrasound anomaly scan is required to assess whether there are any abnormal conditions in the fetal development.