Down screening is between 15-20 weeks of pregnancy, the best time recommended is around 16-18 weeks of pregnancy, this stage once detected will allow time for follow-up tests. Down screening we stipulate is between 15-20 weeks of pregnancy, the recommendation is around 16 weeks of pregnancy, it is in the middle of the overall projected Down screening high risk and low risk, if there is a problem with the Down screening found at 16 weeks of pregnancy, the next step will be further management. For example, non-invasive DNA or amniocentesis, these follow-up operations will leave a certain amount of time, because if you find a problem with the Down’s syndrome, it takes time for further testing, it may be three or four weeks before you can go for amniocentesis, and after amniocentesis, you have to do a culture, and it may be almost four weeks before you can get the results of the amniocentesis. The overall calculation may take two months, if the amniocentesis is done at 21 weeks, another two months time has almost reached 28 weeks, the child has any problems and then go to do an operation to induce labor is already relatively large. Therefore, if the Down’s syndrome is done too late, it may cause problems in the subsequent treatment, and if the Down’s syndrome is negative, there may not be any subsequent troubles.