There are many tests that a pregnant mother’s doctor will prescribe from pregnancy to delivery, and one of the very important tests is the Down’s syndrome screening, which is a must for pregnant women. If you ask why? Well, let’s start by understanding Down syndrome, for which prenatal screening is performed. Down syndrome (Trisomy 21) Normally, there are 23 pairs of chromosomes in a human being, and the 21st pair of chromosomes in a fetus is one more than normal, called trisomy 21, or Down syndrome, also known as congenital stupidity, which is the most common chromosomal abnormality syndrome. It is the most common chromosomal abnormality syndrome. The main manifestations are severe mental retardation, special facial features and structural deformities, which are serious birth defects. Down syndrome is present in about one out of every 700-800 births. Down syndrome is caused by abnormal chromosome number and there is no treatable method, so it is especially important to carry out effective prenatal diagnosis. Currently, the main clinical prenatal diagnostics include maternal blood sampling for Down’s syndrome screening, noninvasive DNA; and invasive procedures including amniocentesis or umbilical vein puncture for karyotyping. Some studies have reported that amniocentesis and cord blood puncture have a 0.1% to 0.5% and 1.1% to 7.0% chance of causing danger to the mother and fetus, or even leading to fetal loss, respectively. Down’s syndrome screening The risk factor for the possible birth of a fetus with congenital defects is calculated by taking the serum of the pregnant woman and testing the concentration of alpha-fetoprotein (AFP), human chorionic gonadotropin (β-hCG) and free estriol in the maternal serum, together with the expected date of delivery, weight, age and gestational week at the time of blood collection. The high risk cut-off value for Down syndrome is 1:275, the MOM for ONTD (open neural tube dysmorphism) is ≤2.5, and the high risk cut-off value for trisomy 18 is 1:260. for pregnant women at high risk for Down syndrome (risk cut-off value for Down screening ≥1:275) How to determine after screening If a pregnant woman is determined to be at high risk for Down syndrome after screening, further testing, such as ultrasound, is required to verify gestational week, and further calculation of her risk value. Typically, the accuracy rate for Down syndrome is 60% to 75%, while the accuracy rate for NTD and trisomy 18 is 80% to 90%. If there is a large error in the gestational week during the screening and calculation of the indicators, the accuracy of the screening results will be significantly reduced. There are false positives and false negatives in Down’s syndrome screening, i.e., a high risk of Down’s syndrome does not necessarily mean that the patient has Down’s syndrome, and a low risk of Down’s syndrome does not necessarily mean that the patient does not have Down’s syndrome. If a high risk rate for Down’s screening is indicated, further ultrasound is needed to verify the gestational week and again calculate if it is still high risk, the patient needs to be advised to undergo amniocentesis to confirm the diagnosis. By examining the fetal decidual cells in the amniotic fluid, Down’s syndrome can be accurately detected. Indications As pregnant women age, there will be a significant increase in the number of chromosomes that do not segregate during germ cell meiosis. Therefore, for pregnant women over 35 years of age, Down’s syndrome screening is not recommended, but rather a direct amniocentesis test. The following is the Down’s syndrome screening report 21-trisomy result 1/267, which is high risk and amniocentesis is recommended: