Prenatal screening is the screening of fetuses that may have certain congenital anomalies through economical, easy and non-invasive testing methods. The purpose of prenatal screening is to minimize the birth of abnormal fetuses. The fetus is susceptible to genetic and environmental factors during growth and development, some of which result in congenital defects of varying degrees. Among live births, common congenital defects due to chromosomal abnormalities include trisomy 21, or Down’s syndrome (DS), also known as congenital stupidity, which accounts for about 1/800 to 1/600 of live births, followed by trisomy 18, which accounts for about 1/3500 to 1/8000 of live births. In addition, neural tube abnormality is also a congenital defect disease with high incidence and serious consequences in China and the world, with a national average incidence of 3/1000. It is preventable but not curable. The main clinical manifestations of trisomy 18 are craniofacial, thoracic and abdominal, and limb malformations, for which there is no effective treatment. It is a common cause of death in the first few weeks of life. Fetal neural tube malformation is mainly manifested as anencephaly, cerebral bulge, cerebrospinal membrane bulge, spina bifida, etc. It is the main cause of stillbirth and infant death in China, closely related to genetic and environmental factors, with complex causative factors. Even if some of the children survive, they need to undergo multiple surgical operations at great expense, and are often left with serious sequelae, bringing a heavy burden to society and families. Due to the high incidence of the above three congenital defects in live births, the survival rate after birth is low, and even if they survive, they cannot live normally, which seriously affects the quality of birth of our population. In order to improve the quality of our population, prevention and reduction of congenital birth defects have become important tasks in perinatal eugenics and prenatal diagnosis at present. It is advocated that every pregnant woman should undergo two prenatal screenings in the early and middle stages of pregnancy to assess the risk rate of congenital dysmorphic children, and systematically and simultaneously analyze and assess the risk rate of conception of trisomy 18 and the risk of open neural tube malformation. The screening was performed by taking maternal serum for PAPP-A+Freeβ-HCG+NT and AFP+hCG at 14-20 weeks of gestation, and assessing the risk of Down syndrome, trisomy 18 and open neural tube defects by risk assessment software, taking into account maternal age, weight, race, gestational week, previous medical history and ultrasound results. The risk of neural tube defects will be assessed by the risk assessment software. Please pay attention to the accuracy when you fill in your personal information, for example, you should fill in your actual age, some pregnant women write their ID card age; the date of your last menstrual period should be filled in according to the Gregorian calendar, if you write the date in the lunar calendar, the result will be inaccurate. It is important to fill in the exact week of pregnancy, otherwise it will also affect the accuracy of the report. If you do not remember your last menstrual period or your menstrual cycle is irregular, your doctor will estimate the gestational week based on information such as the size of the gestational sac, top-rump length and biparietal diameter from the earliest obstetric ultrasound, as well as the earliest time you felt fetal movement and the height of your uterus. Of course, this is only a prediction of the level of risk, not a definitive diagnosis. Prenatal screening is far from ideal. The percentage of high-risk pregnant women with real problems is about 2-3%, and it can only detect 60-70% of children with trisomy 21, which means that 30-40% of trisomy 21 will be missed; at the same time, it also has a 30% miss rate for trisomy 21 children. There are 2 types of results: low risk or negative; and high risk. A screening result of “high risk” only indicates that your fetus has a high chance of developing the disease, but it does not necessarily mean that the fetus is abnormal, but further fetal chromosome examination is needed. If your screening result is “low risk”, it means that your fetus has a low chance of developing these disorders, but it is not necessarily impossible. If a high risk of trisomy 21 or trisomy 18 is reported, further genetic counseling or prenatal diagnostic testing is recommended to confirm the diagnosis, such as non-invasive DNA, amniocentesis, or cord vascular aspiration. If a high risk of neural tube defects (OSB) is reported, as ultrasound can accurately detect neural tube defects, ultrasound for fetal macrosomia can be done at 16-24 weeks of gestation for further clarification and these high-risk tests are not needed. The following is a description of non-invasive DNA, amniocentesis, and umbilical cord vascular puncture. The common advantage of these three methods is that they can directly obtain fetal chromosomes, and the test results are more certain, the main differences between the three are as follows: 1. Amniocentesis: the test is performed between 18 and 24 weeks of pregnancy, and the miscarriage rate is about 5 per 1,000. However, if the amniotic fluid contains fewer fetal cells, or if the cells are not growing well and there are more dead cells, the chimeric child may be missed. Cord blood puncture: The advantage is that it has the highest accuracy, high cell content, fast growth rate, fast results, and high purity, and is not easily contaminated by other maternal cells, which is better than the fetal cells in amniotic fluid. After 24 weeks of gestation, the fetal cells in amniotic fluid start to decrease, and if amniocentesis is performed again, there is a possibility that the results will not be obtained. Therefore, it is recommended to take the fetal cord blood for testing after 24 weeks of gestation. The risk of miscarriage with cord blood aspiration is about 1% and is more complicated than amniocentesis. Therefore, we generally try to avoid performing cord blood puncture and try to have the patient receive prenatal diagnosis before 24 weeks of pregnancy. 3.Non-invasive DNA testing: After 12 weeks of pregnancy by taking 5~10ml of venous blood from the mother and then enriching the fetal DNA in the venous blood for examination. The benefits are no risk of miscarriage and quick results (10-14 days). This technique allows 97% of pregnant women with high risk of prenatal screening to avoid the risks associated with amniocentesis. The accuracy rate is 99%, but admittedly not as high as that of amniocentesis and cord blood puncture, and non-invasive DNA testing does not guarantee detection of fetuses with sex chromosomal abnormalities. There are also some chromosomal abnormalities such as structural abnormalities of fetal chromosomes, such as Robertson ectopic and balanced ectopic, which can be detected by conventional karyotyping but not by noninvasive DNA technology. However, the detection of such abnormalities is not very significant. The incidence of this type of abnormality is low and is not related to fetal intelligence, but mainly manifests as an increased risk of miscarriage when the fetus has the next generation in adulthood.