Pre-natal tests

Pre-natal tests : What are they ? 

Various biological tests are available to the doctor to assess the risk of a chromosomal abnormality as well as neural tube closure abnormalities.

A pathological result shows an increased risk of the foetus having a chromosomal abnormality (chromosome in 3 copies instead of 2) or a neurological problem during the development of the embryo.

The best known chromosomal abnormality is trisomy 21, formerly known as Down's syndrome after the British doctor who described it.

The oldest tests combine a blood test with the measurement of certain markers and measurements taken during an ultrasound scan of the foetus.

A risk is then calculated by combining the markers and relating them to the patient's age. This is not a diagnostic test but an assessment of the risk of a chromosomal anomaly. Above a certain threshold, the risk is sufficiently high for the obstetrician to propose additional analyses.

Today, there are several types of tests.

  • A blood test carried out between the 10th and 13th week of pregnancy with a PAPP-A (Pregnancy Associated Plasma Protein-A) and hCG assay combined with an ultrasound scan enables this risk to be assessed.

  • Using a blood sample taken at around the 15th or 16th week of pregnancy, 3 substances are measured :

    _ AFP (alpha-foetoprotein produced by the foetus)
    _ hCG (human chorionic gonadotropin, a hormone synthesised by the placenta) 
    _ Oestradiol (oestrogen produced by the foetus and the placenta)

    Similarly, a calculation estimates the risk of anomaly. 

  • This is a two-step screening test that measures the maternal blood concentration of PAPP-A (Pregnancy associated plasma protein-A) produced by the placenta, in addition to the triple test. It requires two blood tests and the results are provided after the second blood test. 

    For several years now, a test based on direct fetal DNA research has also been offered.

  • NIPD is a highly effective screening test for the most common fetal chromosomal abnormalities, namely trisomy 21, 13 and 18. The test, which has become widely used in recent years, is rapid, reliable and safe for the foetus. 

NIPT : How does it work ? 

Fetal DNA passes into the maternal circulation during pregnancy. The test therefore consists of analysing this foetal DNA, which is easily accessible by a simple blood sample from the mother-to-be. In general, there is sufficient fetal DNA from the 12th week of pregnancy.

High-throughput sequencing techniques (NGS for Next Generation Sequencing) make it possible to detect the presence or absence of trisomy.

This test is very effective in detecting trisomies. However, in the event of a positive result, a karyotype of the foetus must be carried out via amniocentesis. This is the only definitive test that can confirm or deny the presence of trisomy.

In very rare cases (less than 1%), the test can give a false positive result, i.e. despite a positive result, the foetus does not have trisomy. 

This is why this invasive examination is still necessary in the event of a positive result from the NIPT test.

What the NIPT test does not show : 

_ Low mosaic trisomy in the foetus, i.e. only a fraction of the cells have trisomy
_ Very small anomalies called micro-deletions or micro-duplications
_ Single-gene anomalies (e.g. cystic fibrosis or fragile X syndrome)
_ Sex chromosome anomalies (e.g. Turner syndrome or Klinefelter syndrome)

In case of ultrasound abnormalities in the foetus, the doctor will prefer an invasive examination such as amniocentesis.

The NIPT test must be prescribed by a doctor who will complete the appropriate application and consent forms to be signed.

In summary : 

_ NIPT is a rapid and 99% reliable test
_ It is done after the 12th week, on medical prescription

NIPT detects the following information : 

_ The presence of an abnormal number of copies of chromosome 21, 18 or 13 in the foetus
_ The sex of the unborn child
_ A positive FIPD result indicates a strong suspicion that the fetus is a carrier of trisomy 21, 18, 13. An invasive diagnostic sample will be taken to verify the results
_ A negative result indicates an extremely low risk but does not formally exclude the presence of the anomalies sought

This is a very specialised analysis. 
We work with expert laboratories. 

Ketterthill pregnancy booklet

Download the full Ketterthill pregnancy booklet and find all the details of your analyses during pregnancy ! 

Do you want to carry out this test ? 

Visit one of our medical analysis laboratories.
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