1. Which is more accurate, Early or Mid-Tang?
As a screening test, neither the early nor the mid-test can confirm the diagnosis, so there is no accuracy. The detection rate and false positive rate are usually used to evaluate a certain screening method. A high detection rate and a low false positive rate are the criteria for a good screening tool. From the current situation in China, the detection rate of early tang is higher than that of middle tang, and the false positive rate is lower than that of middle tang. The detection rate of early tang is about 85% and the false positive rate is about 3%, while the detection rate of middle tang is 65-75% and the false positive rate is 5-8%.
2. Why don’t many places do it if the early Tang is more accurate?
NT testing requires the best ultrasound equipment and the highest level of ultrasound doctors, and there are not many doctors in China who are certified by the British Fetal Medicine Foundation for NT. If the baby’s position is not good, the best detection plane cannot be obtained, and the NT measurement will not be accurate, and generally an NT measurement takes at least 20 minutes, so NT testing is the main bottleneck that cannot be carried out on a large scale.
3. Do I still need to have a mid-tang after having a pre-tang?
No. Both early and intermediate NT are screening for the risk of Down’s syndrome, but the practice varies depending on the screening strategy. If you have a single screening, you don’t need to have a mid-test after having an early screening; if you have a combined early and mid-term screening strategy, you need to have a mid-test after having an early screening and then calculate the combined risk.
4. The result of the Down’s syndrome screening is high risk, but since the false positive is not low, is it okay if I want to repeat the test again?
The principle of Down’s syndrome screening is not to repeat the test, because screening is not a diagnosis, but a general judgment of the risk level, and different testing systems may have different judgment on the same sample. Repeat testing can also bring confusion in interpretation, if the results of two tests are different, which one should you believe?
5.Why should I do it if the detection rate is not high, only about 65%?
Although the detection rate is not satisfactory, the detection rate of Down’s syndrome is only 30% if you do not do the screening test and only use the screening test for pregnant women over 35 years old, so something is better than nothing.
6. Is it impossible to do Down’s syndrome screening for twin pregnancies?
It is not recommended to assess the risk of Down’s syndrome in twin pregnancies by maternal serologic indicators alone (e.g. Mid-Tang), but early screening for Down’s syndrome in early pregnancy combined with ultrasound markers of each fetus (including NT, tricuspid regurgitation, etc.) and maternal serologic indicators is valuable.
7.What should I do if I have a low risk of Down’s syndrome with soft ultrasound indicators such as strong ventricular spots?
Ultrasound should first exclude the presence of combined fetal structural malformations and other genetic markers. In case of isolated ultrasound markers, the need for invasive prenatal diagnosis should be decided based on the likelihood ratio (i.e. increased risk of Down’s syndrome) and background risk of different ultrasound soft markers, and the risk value of Down’s syndrome should be corrected again.
8. What if Down’s syndrome screening suggests a critical risk?
Different screening modalities may suggest different risk classifications, some screening strategies are only low risk and high risk, and invasive prenatal diagnosis (e.g. amniocentesis) is recommended for high risk, some screening strategies have critical risk results, and joint screening strategies or recalculation of risk based on ultrasound markers should be considered on a case-by-case basis.
9. Do I need to do amniocentesis if I fail the glucose screening?
No. “Sugar screening” and “Down screening” are different, “sugar screening” is diabetes screening and “Down screening” is Down’s syndrome screening. The latter is a high-risk test that requires an amniocentesis, while the former is a gestational diabetes test that doesn’t need to be done at all. You can’t laugh, but clinically, we have really encountered mothers-to-be who failed the “sugar screening” and asked for amniocentesis, so we really fainted!
10, 35 years old can not do Down screening, must do amniocentesis?
The maternal and child health law in China stipulates that pregnant women older than 35 years old are recommended to undergo prenatal diagnosis (such as amniocentesis) to confirm whether they are carrying a child with Down syndrome. However, it does not mean that you cannot do Down’s screening at the age of 35, and older pregnant women can still do it after fully understanding the value of Down’s screening (i.e. Down’s screening is a risk assessment, and low risk means less likely to have a child with Down’s syndrome, but it does not mean no risk.) After that, you can still do Down screening.
11. We are a normal couple and there is no Down syndrome in our family, why do I need to be screened for Down syndrome?
About 95% of people with Down’s syndrome have normal parents and no Down’s syndrome in the family. The occurrence of Down’s syndrome is due to early cell division errors in the fertilized egg or errors in germ cell (sperm or egg) division. In less than 5% of cases, Down syndrome is associated with an abnormal chromosome structure of the parents (e.g. translocation). Therefore, theoretically, all pregnancies should be screened for Down’s syndrome, regardless of family history, because Down’s syndrome can occur in any normal pregnancy, and the risk of Down’s syndrome in normal pregnant women under 35 years of age is 1/700-1/800.
12. Since Down’s screening is not accurate and non-invasive fetal DNA testing is more accurate, why not just replace Down’s screening with non-invasive?
1) The current non-invasive fetal DNA test only targets aneuploidy of chromosome 21, 13 and 18, while Down’s screening also has a certain screening effect on aneuploidy of other chromosomes and some chromosomal structural abnormalities; 2) Some serological indicators in Down’s screening are associated with the occurrence of certain pregnancy complications and have early predictive value for pregnancy complications (such as pre-eclampsia); 3) From the perspective of health economics From the perspective of health economics, the current cost of noninvasive fetal DNA testing is relatively high and it is not yet suitable to be carried out as a first-line screening tool.
13.What is the difference between non-invasive fetal DNA test and amniocentesis?
Non-invasive fetal DNA testing is performed by measuring the relative content of target regions of DNA (e.g. chromosomes 21, 13 and 18) from the fetus in maternal peripheral blood to determine whether there is an altered dose of these chromosomal segments (e.g. addition or deletion), and does not reveal all information about the fetal chromosomes. Amniotic fluid cells contain cells shed from the fetus, and culturing these shed cells allows visualization of the fetal chromosome bodies and can detect chromosome number and structural abnormalities. Non-invasive fetal DNA testing is currently an advanced screening test for Down’s syndrome, with a detection rate of approximately 99% and a false positive rate of less than 1%. Amniocentesis, on the other hand, is the gold standard for prenatal diagnosis. Non-invasive cannot replace amniocentesis. Amniocentesis is still recommended for non-invasive results with high risk.