Prenatal genetic testing: What do you need to know about these tests?


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Prenatal genetic testing aims to provide prospective parents with information on whether their foetus has genetic disorders. This test, also called non-invasive prenatal testing, is a method that helps determine the risk of the foetus having genetic abnormalities, and is performed using small pieces of DNA from the foetus circulating in the pregnant woman’s blood. Find out in this article how to do it and what to consider before doing it.

How does prenatal genetic testing work?

While it is true that most of the DNA a person possesses is found within the nucleus of the cell, the fragments to be tested in non-invasive prenatal testing circulate freely. These fragments, corresponding to foetal cell-free DNA (cfDNA), usually containing less than 200 DNA building blocks (base pairs), appear when cells die, break down and their contents are released into the bloodstream.

During pregnancy, the mother contains a combination of free DNA from her own cells and those of the placenta in her bloodstream. As we know, the placenta acts as an intelligent filter, supplying oxygen and nutrients to the baby and removing waste produced by the foetus. Interestingly for clinical diagnosis, the DNA of placental cells is often identical to that of the foetus. By analysing the cfDNA of the placenta, some genetic abnormalities can be detected early without harming the foetus.

What is the use of this test?

Non-invasive prenatal genetic testing is often used to look for chromosomal disorders caused by the presence of an extra or missing copy (aneuploidy) of a chromosome. It mainly looks for Down syndrome (trisomy 21, caused by an extra chromosome 21), trisomy 18 (caused by an extra chromosome 18), trisomy 13 (caused by an extra chromosome 13) and extra or missing copies of the X chromosome and Y chromosome (the sex chromosomes). The accuracy of the test varies according to the disorder.

In addition, this test may include the detection of additional disorders, caused by missing (deleted) or duplicated (copied) sections of a chromosome. Among the uses of these tests is also the detection of genetic disorders that are caused by variants in single genes.

As technology advances, the cost of performing these tests is decreasing and, increasingly, researchers believe that the tests will be available for more genetic conditions.

How is it done?

As this test only requires blood to be drawn from the pregnant woman and poses no risk to the foetus, prenatal genetic testing is said to be non-invasive. These tests are screening tests, i.e. they will not give a diagnostic result and can only estimate whether the risk of having certain genetic conditions increases or decreases.

Non-invasive methods are multiple, although the most common is to count all cfDNA fragments, both fetal and maternal. If the percentage of cfDNA fragments of each chromosome is as expected, then the foetus has a lower risk of having a chromosomal condition (negative test result). If the percentage of cfDNA fragments of a particular chromosome is higher than expected, then the foetus has an increased chance of having a trisomy condition (positive test result). A positive screening result indicates that further diagnostic tests should be performed to confirm the result.

False positive cases exist when the test results indicate a high risk of a genetic abnormality but the foetus is not affected. The results may also indicate a decreased risk when the foetus is affected (this being a false negative). By analysing foetal and maternal cfDNA, the test can also detect a genetic condition in the mother.

In order to carry out this study, a sufficient amount of foetal cfDNA must be available in the mother’s blood in order to identify chromosomal abnormalities in the foetus. Normally, it should be above 4%, which is usually around the tenth week of pregnancy. If a low fetal fraction is found, this may result in inability to perform the test or a false negative. This may be due to the test being performed too early in the pregnancy, sampling errors or a foetal abnormality.

When to take the test?

A large number of specialists, including the American College of Obstetrics and Gynecology (ACOG) and the International Society for Prenatal Diagnosis, recommend testing for women classified as being at high risk of having a baby with a chromosomal disorder as early as the tenth week of pregnancy.

What sample is required?

Prenatal genetic testing requires a blood sample taken from a vein in the mother’s arm. As mentioned above, the test is called non-invasive because there is very little risk to the mother or baby in obtaining the sample.

Who should do it?

Some parents seek to know in advance if their baby will be born with a genetic disorder, because knowing in advance gives them time to learn and plan for the medical care their child may need. Others may decide to terminate the pregnancy in certain situations and, for a final group, there is no need to know this information before the child is born. It is important to consider that this test is not mandatory at any stage of pregnancy and that each individual can choose whether or not to do it according to their preferences and needs.

AtAmbar Lab we have a diagnostic laboratory where we carry out various personalised analyses interpreted by specialised technicians. Contact us for more information on how to outsource the management of prenatal genetic testing.

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