The risk of trisomy 21 is divided into low risk, critical risk and high risk, while low risk is less than 1 per 1,000 and high risk is greater than 1/270. When a woman is about 16 weeks pregnant, she usually needs to undergo Down’s syndrome screening to confirm the risk of the fetus having Down’s syndrome, and this is mainly done by ultrasound to see the size of the fetus and to make sure that the fetus is alive, and by drawing blood from the mother’s vein to do the Down’s syndrome test. The screening test is based on the free hcg hormone and fetal alpha protein in the blood of the pregnant woman, as well as the age of the pregnant woman and the size of the fetus, to determine the risk of the fetus. Generally Down’s syndrome screening is done in addition to trisomy 21 and also checks the risk of chromosome 18. If the test result indicates that the risk value of trisomy 21 is less than 1 per 1,000, it usually indicates that the risk of the fetus having Down’s syndrome is particularly low, and basically the fetus is considered to be normal and will not have Down’s syndrome. If the Down’s syndrome screening reveals a critical risk or even a high risk, further non-invasive DNA or amniocentesis will be required to confirm the problem.