What Is Maternal Serum Screening?

Down's screening is an abbreviation of Down's syndrome prenatal screening test, which is a special significance test method. The purpose is to determine the concentration of fetal protein A, villous gonadotropin, and free estriol in the maternal serum by testing the blood of pregnant women, and to determine whether the fetus is born with congenital, neurological disorders by combining the age, weight, and gestational age of the pregnant woman. Risk factor of tube defects ^[1] .

Down syndrome is trisomy 21, also known as congenital stupidity or Down syndrome. It is a disease caused by a chromosomal abnormality (an additional chromosome 21). 60% of the children will have an abortion in the fetus, and the survivors have obvious mental retardation, special faces, growth and development disorders, and multiple deformities ^[2] .

Chinese name: Down's screening
Foreign name: Down's screening
Full name: Prenatal screening test for Down syndrome ^[1]

Detection category: Down syndrome
Detection method: Taking serum to detect maternal and fetal proteins ^[1]
Detection significance: Risk factor for judging fetuses from birth defects and neural tube defects ^[1]

Down's screening for Down syndrome

Down'ssyndrome, also known as 21-trisomy syndrome, is the most common form of chromosomal disease in children and is an autosomal aberration. Children are characterized by stunted growth and mental retardation, accompanied by multiple organ developmental disorders or deformities. The incidence of Down syndrome in China is 1/600 to 1/800. On average, one case of Down's child is born in 20 minutes ^[3] .

Down's Screening Program

Down's Screening in Early Pregnancy

Early pregnancy is generally 7 to 13 weeks after pregnancy. At this time, dual screening refers to a combined screening program that uses serum pregnancy-associated protein-A and human chorionic gonadotropin or free -hCG as indicators and calculates the risk of DS in the fetus combined with parameters such as the age of the pregnant woman ^[3] .

Down's screening during pregnancy

The second trimester of pregnancy is 14 to 20 weeks after pregnancy. At this time, the dual screening method uses serum alpha-fetoprotein AFP and human chorionic gonadotropin or free -hCG as indicators, and calculates the risk of DS in the fetus with parameters such as age Joint screening program.

Triple screening is also available during mid-pregnancy. Triple screening refers to a combined screening program that uses serum AFP plus hCG (or free -hCG) and non-conjugated estriol as indicators: calculates the risk of fetal DS by combining parameters such as maternal age ^[3] .

Down's screening screening mode selection

The dual screening in early pregnancy and the triple or triple screening in middle pregnancy are simple and can obtain satisfactory screening results. It has a significant effect on reducing DS births and controlling the incidence of birth defects. According to the specific conditions of mainland China, at the current stage, the two-screen screening during the second trimester is an effective DS prenatal screening program ^[3] .

Down's screening screening indicators

Ultrasound and serological indicators have important positions in prenatal screening indicators, but they also have their own clinical characteristics. Testing time and test results. The combination of these parameters in prenatal screening not only improves sensitivity and specificity, but also makes up for a single item. Insufficient detection ^[3] .

Down's screening ultrasound diagnostic indicators

The ultrasound indicators used for screening for Down syndrome include: neck transparency (NT), nasal bone (NB), length of femur and humerus, ventricular highlights, and cardiovascular malformations ^[3] .

1. Neck transparent layer (NT)

The cervical transparent layer refers to the maximum thickness of the cervical spine between the horizontal sagittal plane and the subcutaneous soft tissue of the skin. NT is also called the cervical zona pellucida scan. By measuring the thickest part of the ultrasonic echoless layer of the fetal neck without echo, Down syndrome fetuses have subcutaneous hydrops. The neck skin is relatively thick, so it is used to evaluate the fetus. Whether it is possible to have Down syndrome is tested in a way ^[3] .

2. Nasal bone dysplasia

Due to the peroxidative reaction of the extracellular matrix of Down's syndrome fetus, the development of the nasal bone is affected, so the fetal B-ultrasound failed to detect the presence of the nasal bone ^[3] .

Cardiovascular malformations

More than half of Down's syndrome fetuses are associated with cardiovascular malformations, and more than 90% of heart malformations occur early, so cardiac ultrasonography in the first trimester is particularly important ^[3] .

Down's screening serology

At present, Tang screening tests are used to test alpha-fetoprotein (AFP), free estriol (uE3), human chorionic gonadotropin (F-HCG), and pregnancy-related plasma protein A (PAPP-A) in pregnant women's blood. Among them, alpha-fetoprotein (AFP), free estriol (uE3), and human chorionic gonadotropin (F-HCG) are often used for combined detection.

1. Alpha-fetoprotein

A type of fetal protein (AFP) is a specific globulin of the fetus, with a molecular weight of 64,000 to 70,000 Daltons. It may have immunoregulatory functions of glycoproteins during pregnancy, which can prevent the fetus from being rejected by the mother. AFP is synthesized by the yolk sac in the first 1 to 2 months of pregnancy, followed by the fetal liver. The fetal digestive tract can also synthesize a small amount of AFP to enter the fetal blood circulation ^[4] .

The fetal blood AFP value increased rapidly at 6 weeks of gestation and reached a peak at 13 weeks of gestation. After that, it gradually decreased to full-term as the pregnancy progressed. The AFP in the amniotic fluid mainly came from fetal urine. Amniotic fluid and fetal blood are not consistent with the changes in amniotic fluid and fetal blood ^[4] .

In early pregnancy, the concentration of AFP in maternal blood is the lowest, and it gradually increases with the progress of pregnancy. It reaches a peak at 28 to 32 weeks of pregnancy and then decreases. Pregnant women with a congenital fetus have a serum AFP level of 70% of normal pregnant women, which means that the average MoM value is 0.7 to 0.8 MoM ^[4] .

Human chorionic gonadotropin

Pregnant women with congenital fetuses have an increase in serum F-HCG levels, with an average MoM value of 2.3 to 2.4 MoM. hCG is a human chorionic gonadotropin synthesized by placental cells and consists of two subunits, a- and b-. HCG exists in two forms, complete hCG and individual b-chains. Both HCGs are active, but only HCG present in the b-single-chain form is a specific molecule for the assay. HCG enters the mother's blood after fertilization and rapidly proliferates until the 8th week of pregnancy, then slowly decreases the concentration until the 18-20th week, and then remains stable. The MoM value is a ratio, that is, the detection value of a pregnant woman's body divided by the median value of a normal pregnant woman at the same gestational week, which is the MoM. Because the level of prenatal screening will change greatly with the increase of gestational weeks, its value must be converted into a multiple of the median (MoM) representation to make it "standardized" for clinical judgment ^[4] .

3.Free estriol

Unconjugated estrion (uE3) is mainly synthesized by the fetal-placental unit, so as the gestational age increases, the fetal placental function gradually increases, and the synthesis of free estradiol and triol also increases, which decreases sharply after delivery. Therefore, the determination of pregnant women's serum estriol can be used to detect the fetal growth status in the second trimester. The maternal serum uE3 level of DS is lower than normal 30% or more, so the maternal uE3 can be estimated to be at risk for DS ^[3] .

4, pregnancy-related plasma protein-A

Pregnancy-associated plasma protein-A (PAPP-A) is a macromolecular glycoprotein produced by the placental syncytiotrophoblast and decidua. Its gestational level increases with the gestational week until delivery. During abnormal pregnancy in the first trimester, the level of PAPP-A is significantly reduced, and there is no difference between normal and normal levels in the second trimester. Therefore, PAPP-A is an important serological indicator for screening fetal chromosomal diseases in the first trimester ^[3] .

Down's screening test

Down's screening villus sampling

Villous sampling is an invasive test in the first trimester of pregnancy, and some cell sample tissues are obtained from the developing placenta, because the fetal and placental tissues are derived from the same cells. It can provide fetal chromosomal abnormalities, such as Down's disease or other genetic conditions ^[3] .

Down's screening amniocentesis

Amniocentesis is a invasive test in the second trimester of pregnancy. The karyotype analysis of fetal cells and the enzymatic detection are performed by culturing cells that have fallen off from the amniotic fluid. Generally, 20 ml amniotic fluid is drawn to culture the cells that fall off from the fetus (there may be risks of infection, leakage of amniotic fluid, abortion, etc.), and the chromosomes of the cells are tested (21 fetus chromosomes are tested). The rate is 100% ^[3] .

DNA Down's Screening for Noninvasive DNA Detection Gene

Non-invasive DNA detection of genes is a new technology, which needs further research. The principle is that the fetal cells will penetrate into the mother's blood through the placenta, and will be destroyed by the mother's immune system, and the fetal DNA will remain. The mother's blood will be used to detect the fetal free DNA and the risk of the fetus from Down syndrome will be obtained. ^3] .

Clinical significance of Down's screening

Down's screening is performed by taking the serum of pregnant women, detecting the concentrations of fetal protein A, villous gonadotropin, and free estriol in the maternal serum, and calculating the birth rate by combining the expected date of birth, age, weight, and gestational week when blood is collected Risk factor for birth defects. The hospital generally stipulates that it takes place within 14-20 weeks. If the results of the Tang screening test indicate that the fetus has a higher risk of Down syndrome, a more definitive diagnosis should be performed-amniocentesis or villus examination ^[5] .

Down syndrome is an occasional disease, so every pregnant woman may have a "Down's child." Although many people have questioned the Down's screening, although the results of the Tang's screening cannot accurately determine whether the baby is stupid, after all, this is the easiest way to determine whether the baby is stupid and does not damage the fetus. It is recommended that every pregnant woman Attention must be paid to screening for Down syndrome ^[5] .