Mothers-to-be may have heard of Down’s syndrome screening at one point or another, and may know that Down’s syndrome screening is a test to detect whether a newborn has Down’s syndrome. Down syndrome, also known as trisomy, is a disease that occurs during pregnancy due to chromosomal abnormalities (i.e., one extra chromosome) in the developing fetus. Children with Down syndrome often have a special face and abnormal brain development, especially the nervous system, resulting in mental retardation, inability to take care of themselves, and complex cardiovascular disease, requiring long-term family care, which can cause a great emotional and financial burden to the family. The incidence of Down’s syndrome in the general population is 1/800, and every pregnant woman has the possibility of having a child with Down’s syndrome, and the older the mother-to-be, the more likely she is to have a child with Down’s syndrome. There is no way to detect Down’s syndrome through conventional screening methods during pregnancy, such as B-mode ultrasound. There is no good way to treat children with Down syndrome, and the preventive way is to terminate the pregnancy before the baby is born. Therefore, in order to prevent Down’s syndrome in newborns and to ensure an optimal birth, every mother-to-be needs to be screened for Down’s syndrome during pregnancy. People at risk for Down’s syndrome and the best time to get tested So, when should Down’s screening be done? There are two stages of Down’s syndrome screening, one is early pregnancy Down’s syndrome screening, which is performed around the 11th week to 13+6 weeks of pregnancy, and the detection rate is 85% to 90% when combined with ultrasound. The second is the midtrimester Down’s syndrome screening, which is mainly a serological test performed around the 14th to 20th week of pregnancy, but the detection rate is 60% to 65% because of the ultra-low ultrasound index. The detection rate can be increased to 90%-95% by combining the screening indicators of early and middle pregnancy, but the combined screening must be performed at the same hospital. Therefore, in order to ensure the detection rate, it is recommended that mothers-to-be go to the hospital for early pregnancy Down’s syndrome screening when they are about 3 months pregnant. The detection rate of Down’s syndrome screening is not 100%, but a percentage value. So one wonders how the risk value for Down’s syndrome screening is calculated. The risk of Down’s syndrome screening in early pregnancy is evaluated by the number of fetuses, maternity date, last menstrual period, maternal weight, serum indicators, NT value (nuchal translucency width), etc. It is a comprehensive risk calculation. However, since there is no NT indicator (the fluid under the fetal skin may be absorbed by the developing lymphatic system at this time, so that the exact detection data cannot be obtained), the detection rate will be lower than that of early pregnancy screening. Because Down’s syndrome screening is a comprehensive risk calculation, mothers need to tell the doctor accurate information about themselves during the test, such as the time of their last menstrual period, weight, whether they smoke, whether they have taken birth control pills during pregnancy, etc. If the result shows a high risk of Down’s syndrome, the mother will need to undergo amniocentesis to determine if her baby has Down’s syndrome.