Mothers-to-be are familiar with the term “Down’s syndrome screening”. Down’s syndrome, or trisomy 21, is a chromosomal abnormality (an extra chromosome 21) that causes abortion in 60% of children in the early stages of pregnancy, and those who survive have significant intellectual backwardness, special facial features, growth disorders and multiple malformations. It will bring a great burden and psychological pressure to the country, the society and especially the family. The sixth hospital of Sun Yat-sen University, deputy director of the Department of Obstetrics, Dr. Gao Yu, to avoid the birth of Down’s baby, only by Down’s screening is not enough, generally need “three gates” to keep watch.
STEP1: Initial Screening
Route 1: Serological screening
Detection rate: 65%
The internationally accepted method of Down’s syndrome screening is to assess the risk of Down’s syndrome, trisomy 18 and open neural tube defects at 11-20 weeks of gestation through a combination of serological and ultrasound examinations, combined with the mother’s age, weight, gestational week and medical history.
Many pregnant women get their birth certificate in the third month of pregnancy and wait until the fourth month of pregnancy before they go to the hospital to build a file, at which time they start to do all kinds of screening, which can detect 65% of Down’s fetuses, but if they are confirmed to be Down’s fetuses after prenatal diagnosis, then they will be induced, which is more harmful to the woman’s body because it is already the middle of pregnancy.
Route 2: One-stop Down’s syndrome screening (combined serology + ultrasound)
Detection rate: 87% (there may be 5% false positives)
Therefore, Director Gao Yu recommends that pregnant women who are in a position to do so should go to a qualified hospital at 11-13 weeks to undergo “one-stop” prenatal screening for Down’s syndrome, i.e. serological screening plus NT screening, the latter refers to the use of ultrasound to look at the nuchal translucency layer of the early fetus, more than 3.0 mm indicates the risk of Down’s syndrome. The latter refers to the use of ultrasound to look at the nuchal translucency layer in the early stage of the fetus, more than 3.0 mm indicates the risk of Down’s syndrome, and the two together can detect 87% of Down’s babies with higher sensitivity than serological screening in the middle stage. Once the fetus reaches 14 weeks and the nuchal translucency layer disappears and cannot be seen, it is meaningless to check NT again.
STEP2: Prenatal diagnosis
Generally speaking, for ordinary pregnant women, if the initial screening result of Down’s syndrome indicates “positive”, it is necessary to proceed to the next step – prenatal diagnosis. However, for some high-risk pregnant women (such as those who are over 35 years old, have had a child with chromosomal abnormalities, or have a fetus with a non-fatal abnormality detected during ultrasound, but suggesting a possible chromosomal abnormality), the doctor will recommend her to undergo prenatal diagnosis directly.
Route 1: Interventional prenatal diagnosis
Accuracy: 100%
Doctors will take different prenatal diagnostic methods depending on the pregnant woman’s gestational weeks, including chorionic villus aspiration (11 weeks-13 weeks + 6 days of pregnancy), amniocentesis (17 weeks-23 weeks), and cord blood aspiration (over 23 weeks) to determine whether the fetus has chromosomal abnormalities by culturing fetal cells and doing karyotype analysis. This test has a 100% accuracy rate and is the “gold standard” for the diagnosis of Down’s syndrome fetuses.
However, it has its drawbacks. For example, because the cells need to be cultured, the waiting time for the test results is long. For example, chorionic villus and umbilical cord blood tests take 3 weeks to 1 month to report, and amniocentesis takes a month to 6 weeks for the results. The time spent waiting for the results can be “torturous” for the pregnant woman and her family and can be very anxious. Another disadvantage of interventional prenatal diagnosis is the risk of fetal miscarriage due to the irritation of the uterus with either type of puncture. The risk of miscarriage is 1 percent for amniocentesis and 2 percent for the other two. Gao Yu explained that the doctor’s experience and skill can reduce the incidence of miscarriage caused by interventional prenatal diagnosis to some extent, but it cannot be avoided 100%. There are some pregnant women who are positive for primary screening, but are torn by the fear that prenatal diagnosis will lead to miscarriage because they have had a hard time conceiving a baby.
Route 2: Non-invasive prenatal diagnosis
Accuracy: 99%
Non-invasive prenatal diagnosis is a prenatal diagnosis method that has emerged only in recent years. It only requires a small amount of blood from the mother-to-be, from which a very small amount of fetal DNA is purified, and whole genome sequencing is used to analyze the fetus for chromosomal aneuploidy, with an accuracy of 99% for Down syndrome detection. For advanced maternal age, the use of non-invasive prenatal diagnosis can reduce the risk of fetal miscarriage due to invasive prenatal diagnosis. Director Gao Yu recommends that non-invasive prenatal diagnosis can be done for all pregnancies after 12 weeks, and it is best to do it as early as possible.
This type of prenatal diagnosis is not completely without drawbacks. The whole-genome sequencing method of non-invasive prenatal diagnosis can only detect large chromosomal abnormalities, such as one more or less chromosome, or large segments of chromosomal duplication or deletion, while subtle chromosomal abnormalities, such as chromosomal balance ectopic, cannot be detected for the time being. In addition, if the pregnancy is a multiple pregnancy and the non-invasive test report indicates a high risk, it is impossible to distinguish which fetus is abnormal or both fetuses are abnormal, which also requires invasive prenatal diagnosis to further confirm the diagnosis. The accuracy rate of non-invasive prenatal diagnosis cannot reach 100%, so if the test result indicates a positive result, an interventional prenatal diagnosis is still needed to confirm the diagnosis before deciding whether to give up the fetus.
Director Gao Yu introduced that for pregnant women with high risk of Down’s syndrome (e.g. those with risk of Down’s syndrome above 1/270, age above 40, or fetal malformation clearly seen in ultrasound), doctors will directly recommend her to undergo interventional prenatal diagnosis; for those with critical high risk (risk between 1/270 and 1/1000 in the initial screening), non-invasive prenatal diagnosis can be preferred.
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