Why do we need to do B-ultrasound in mid-term Down syndrome screening?

Female friends need to do Down syndrome screening during pregnancy, and the frequency of testing varies for different groups during pregnancy. Some pregnant women also need to do mid- to late-stage Down syndrome screening after the early-stage Down syndrome screening. So, does the mid- to late-stage Down syndrome screening require B-ultrasound?

Do I need to do B-ultrasound for Down syndrome screening in the middle and late stages?

There is no need for B-ultrasound in middle-aged and late-stage dysmenorrhea, as it does not have much practical significance. You should know that B-ultrasound is mainly used to check the location of the fetus, the location of the embryo, the size of the fetus, whether it is twins, whether it is deformed, and the amount of amniotic fluid in the uterus. It has little to do with Down syndrome screening, so there is no need for B-ultrasound for Down syndrome screening. Only when the screening prompts a high risk is further prenatal examination recommended, namely "amniocentesis" to analyze the fetus's sex chromosomes and ultimately make a diagnosis of whether there is an abnormality. Amniocentesis is also a safe diagnosis method and there is no need to worry. But generally speaking, the decision-making power lies with the pregnant woman herself.

Down syndrome screening can detect 60-70% of Down syndrome patients. It needs to be made clear that the Down screening test can only help determine the chance of a fetus having Down syndrome, but it cannot confirm whether the fetus has Down syndrome. In other words, when the blood test index value is high, the chance of having a baby with Down syndrome is higher, but it does not mean that the fetus must have problems. It seems that older pregnant women over 35 years old have a higher chance of having a baby with Down syndrome, but it does not mean that their fetuses must have problems. On the other hand, even if the test indexes are normal, it cannot guarantee that the fetus will definitely not get sick.

Pregnant women whose Down syndrome screening index value is higher than normal should undergo amniocentesis or chorionic villus sampling. Only if the results of amniocentesis or chorionic villus sampling are normal can the possibility of Down syndrome be 100% ruled out.

Early pregnancy Down syndrome screening NT

What we call "Down syndrome screening" is the mid-term Down syndrome screening, which is usually done by drawing blood for testing around 16 to 18 weeks of pregnancy. In addition to the mid-term Down syndrome screening, the theoretical Down syndrome screening also includes the early Down syndrome screening, which is done by doing B-ultrasound to measure the fetal nuchal translucency (NT) between the 11th and 13th weeks of pregnancy, which is commonly known as "NT test". Early Down syndrome screening can identify whether there are chromosomal abnormalities. During pregnancy, B-ultrasound examination accurately measures the thickness of the fluid under the skin of the baby's neck to determine the risk of the baby suffering from Down syndrome. The general standard value should be less than 3mm. However, it has relatively high technical requirements, and not all companies and doctors can carry out this inspection. The NT test is for 11 to 13 weeks of pregnancy. If the pregnancy is beyond this number, the NT test cannot be performed.

The accuracy of screening methods varies at different gestational weeks. The accuracy of early Down syndrome screening combined with B-ultrasound NT test is about 85%, the accuracy of mid-stage Down syndrome screening (triplet growth hormone) is about 75%, and the accuracy of minimally invasive DNA testing is about 99%. If the Down syndrome screening result is high risk, according to the law and regulations: after the screening result comes out, the next step should be amniocentesis, so that the final result can be obtained. If you choose a minimally invasive DNA test, the result of the test is still a screening result. If the result is still high risk, the child's future still requires amniocentesis to obtain the result.