(a) What is fetal chromosomal abnormality screening? Currently, these screenings can only detect trisomy 21, trisomy 18, trisomy 13 and neural tube defects. (2) How many types of chromosomal abnormality screening are there? 1.Early pregnancy screening: performed between 10 and 13+6 weeks, ultrasound will measure the thickness of the nuchal translucency layer (NT) of the fetus. 2.Mid-trimester screening: performed from 15 to 20+6 weeks, Integrated Screening: early pregnancy screening + mid-trimester screening, resulting in a risk value to determine whether to perform prenatal diagnosis, mainly including: serologic integrated screening and comprehensive integrated screening (serologic integrated screening + NT). Sequential Screening: There are two types of sequential screening: first, Stepwise Sequential Screening, i.e., early pregnancy screening, early pregnancy risk value, prenatal diagnosis is recommended for high-risk patients, low-risk patients receive mid-pregnancy screening until mid-pregnancy, and finally, comprehensive risk analysis is performed based on the results of early and mid-pregnancy screening. The second is Contingent Sequential Screening (CSSS), where the population is divided into three categories after early pregnancy screening, with those with risk values greater than 1/60 being screened for prenatal diagnosis; those with risk values less than 1/1000 not requiring midtrimester screening but only routine physical examination, and those in between being screened for midtrimester screening. Prenatal diagnosis will be determined after midtrimester screening. The difference between sequential screening and integrated screening is that the former has a risk assessment after early pregnancy screening and decides whether to undergo mid-pregnancy screening according to the risk level, while the latter has no risk assessment in the middle, and proceeds directly to mid-pregnancy screening and assesses the risk level according to the results of both screenings. (C) Screening population and how to choose screening items? 1. When the age of pregnant women exceeds thirty-five years, the chance of birth of babies with chromosomal abnormalities increases greatly, therefore, the Technical Standards for Prenatal Screening and Diagnosis of Common Fetal Chromosomal Abnormalities and Open Neural Tube Defects promulgated by the Ministry of Health stipulates that midtrimester maternal serological prenatal screening is applicable to midtrimester pregnant women under the age of 35 for screening. 2. Before choosing which screening program to take, patients should be fully informed about the false-positive and detection rates, advantages and disadvantages, limitations of various screening methods, and the risks and benefits of the diagnostic program. The choice of screening procedure depends on a number of factors, including: the week of gestation at the time of the first prenatal visit, singleton and twin or multiple births, family history, previous maternal history, availability of NT measurement, sensitivity and limitations of screening tests, risk of invasive diagnostic tests, willingness to undergo early pregnancy screening, and willingness to terminate the pregnancy early. (d) What are the advantages and disadvantages of screening tests compared with diagnostic tests? 1. Advantages of screening test: It can identify people at high risk for Down syndrome, trisomy 21 and trisomy 18. The screening test has a higher rate of positive diagnostic tests than those who do not participate in the screening test. The screening test can reduce the number of invasive diagnostic tests and reduce the number of miscarriages and other adverse consequences caused by them. 2. The most important disadvantage of screening tests is that the detection rate is not 100%. Pregnant women and doctors should understand that screening tests provide a risk level rather than a diagnostic result and cannot detect all chromosomal abnormalities. Moreover, the presence of false positives can increase the psychological burden of pregnant women with false positive screening. (E) Noninvasive Prenatal Screening 1. Noninvasive prenatal screening is performed by high-throughput sequencing of free DNA in the plasma of pregnant women, of which 3% to 13% of DNA comes from the fetus after 10 weeks of pregnancy. 2. Non-invasive prenatal screening can only screen out trisomy and sex chromosome abnormalities, and is suitable for pregnant women aged 35 years and above, pregnant women at high risk for ultrasound screening, pregnant women with a history of pregnancy and delivery of babies with chromosomal abnormalities, pregnant women with a family history of chromosomal abnormalities, and pregnant women at high risk for serum screening. The sensitivity of screening for trisomy 21, trisomy 18, trisomy 13 and sex chromosome polyploidy is 99.3%, 97.4%, 91.6% and 91% respectively, while the false positive rate is only 0.2%, 0.2%, 0.1% and 0.4%. Several countries have included it in their screening guidelines, but it is important to note that noninvasive prenatal screening is still a screening method and cannot replace diagnostic testing.