Prenatal screening for trisomy 21: why a blood test?

A new prenatal blood test for Down’s syndrome is now available in Germany, after Switzerland and several other European countries. What is it exactly?

Yves City: When a woman is pregnant, fragments of DNA from the fetus pass into her placenta. More and more sophisticated means now make it possible to sequence these fetal cells circulating in the maternal blood, in order to find specific sequences of chromosome 21. By taking other control chromosomes, researchers are able to tell if there are the same amount of chromosomes 21 as other chromosomes. Even if it is also a blood test, this test has nothing to do with the dosage of “serum markers” which seeks to assess a risk of Down’s syndrome in the future baby.

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Who is this test for?

YV: So far, this test has always been performed in women whose first trimester screening showed a high risk of Down’s syndrome and who were between 21 and 12 weeks of gestation. Therefore, if we want to try to expand its use right now, we have to start with this group. And stop there. The College of Gynecologists and Obstetricians has also reaffirmed that this genetic test was not intended to be used in all pregnant women, but only in those whose combined screening organized in the first trimester by ultrasound and serum markers, correlated with maternal age, had shown an increased risk of having a child with trisomy 21.

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How does this innovation constitute a breakthrough in prenatal care?

YV: The objective of this test is to allow women, who are at so-called high risk through screening, not to have to undergo amniocentesis or a trophoblast biopsy. These invasive procedures cause miscarriages in 0,1% of cases. This risk is thus avoided by bringing about the same result by taking a blood test. However, it should be noted that this is a test which is intended only for the diagnosis of trisomy 21. If a woman is at high risk because on ultrasound, for example, the neck of the fetus is a little thick, it may not be trisomy 21. Other abnormalities may be. origin of an ultrasound image. In this case, this test is not appropriate. This new tool does not completely replace all amniocentesis.

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Does this non-invasive prenatal screening have its place in France?

YV: For now, this test has a small place in terms of indication, but a very important place in terms of stake, since once again, he would avoid miscarriages which are still too frequent. In France, prenatal screening is highly regulated. One of the advantages of this system is that it is supposed to first be offered and explained. In no case is it systematic. The fact of having a blood test can somehow trivialize this screening. Be careful. The dimension of information and free choice is just as important whether a blood test or an amniocentesis is done.

But this test also comes at a cost *. The question is whether, in France, we want to include in the pregnancy monitoring a blood test only for the diagnosis of trisomy 21, which affects “only” 1 in 1000 pregnancies overall. Is this where we want to put state resources back? This economic issue is quite blocking, since until now the Biomedicine Agency and the National Ethics Committee have never spoken out to authorize tests that would be paying. It is unthinkable for us to charge for these exams, because this creates a disparity in access to care. This would mean that we agree to include it in the prenatal arsenal, that this test is thus supported. Personally, it seems extremely legitimate to me to integrate it and start using it in women at risk. Afterwards, if a woman who has no particular risk wishes to take this test, blockages will appear. Under these conditions, it is the ethics of prenatal screening in France that is at stake.

* Since December 27, 2018, the screening test is now reimbursed.