What is prenatal genetic testing ?
Pregnancy is a very emotional and busy time for the expectant mother. She needs to come to terms with her changing body and prepares to welcome a whole new being into the world. One of the most important facets of this preparation is prenatal care. The expectant mothers have to make regular visits to their doctors and undergo a number of medical procedures.
The tests are the instruments that make monitoring the well-being of both the mother and the fetus possible. Still, as the fetus is hidden in the womb during development, it is not easy to be absolutely sure of its health. How to be really sure that there are no problems – and to be ready if some kind of health condition is indeed present ? Modern prenatal genetic testing can help with that, and here is how it becomes important.
As the name suggests, prenatal genetic testing during early pregnancy involves analyzing the genetic material of the fetus. These tests can be complicated to perform, since the fetus is protected by the mother’s womb and it is challenging to get any testing material from it. Despite this, there are two types of prenatal genetic testing options available, namely:
1) Prenatal genetic screening: This refers to testing that can potentially identify whether the fetus could have certain birth defects; many of which are genetic diseases or genetic disorders. This genetic screening is offered during the first or second trimester of the pregnancy, and most often involves using non-invasive prenatal genetic testing procedures. This type of testing provides a suggestive finding of a genetic disease or genetic condition, but not a definitive answer on the diagnosis of a genetic disease or genetic condition in the fetus during early pregnancy.
2) Prenatal diagnostic testing: This testing is often done to confirm or refute findings obtained from prenatal genetic screening. This type of testing provides a conclusive diagnosis of a genetic disease or genetic disorder in the fetus during early pregnancy, and involves using invasive prenatal diagnosis procedures.
What genetic disorder can arise in the fetus during the pregnancy ?
There are thousands of clinically characterized genetic diseases or genetic disorders. However, factors such as the age of the parents, family history of genetic disorders as well as personal medical history of genetic disorders can influence the risk of a genetic disease or genetic disorder in the developing fetus during early pregnancy. However, on rare occasions a de-novo genetic disease or genetic disorder can arise in the developing fetus irrespective of neither parents having a personal medical history or even a family history of such genetic disease or genetic disorder.
During pregnancy, everything has to go according to the plan. The fertilized egg contains hidden instructions in the form of genes which determine what you are physically and mentally. Genes are special fragments on a very long, complex molecule called DNA. As our bodies are very complex, our DNA molecules are also very long and they are packed into structures called chromosomes. Chromosomes in most of the cells of the body are arranged into pairs – one chromosome comes from the mother, and the other from the father. There are 24 chromosome pairs in total for a human being: 23 pairs of chromosomes that are responsible for most of our features and development, and another pair determines our gender, X and Y. This last pair is called sex chromosomes. Geneticists usually assign a number to each pair for convenience, and they know what genes are located on each of them. Geneticists also know what genetic diseases or genetic disorders are associated with a chromosome, as well as with some individual genes.
Sometimes, a male sperm or a female egg can form without one of the chromosomes, or with an extra chromosome or even have structural problems within a chromosome. This often results in the developing fetus manifesting stereotypical genetic disorders that are associated with chromosome aberrations; and are referred to as chromosome syndromes. A mistake can also occur in one of the genes in the egg or sperm. And if such egg or sperm participates in fertilization, it is likely to contribute to a developing fetus which acquires this genetic mistake. There are also other mechanisms of how genetic disorders or genetic diseases can arise in the developing fetus. If the genetic mistake in the developing fetus is too severe, the fetus would probably be miscarried. However in general, some genetic mistakes can have little effect on the baby’s future, while others can be a serious threat to its life.
What is non-invasive prenatal genetic testing ?
Non-invasive prenatal genetic testing, also known as NIPT, refers to testing cell free DNA of the fetus that circulates in the mother’s blood for genetic diseases or genetic conditions. Our blood contains many components: proteins, lipids, multiple types of cells. There can be small traces of DNA as well. This DNA is called cell-free DNA (cfDNA). These DNA molecules are usually short and probably appear in the blood when cells of the body die; and of which some of it contains DNA from the developing fetus that is circulating with the mother’s blood. This discovery is crucial since it means that one can check the health of the developing fetus during early pregnancy by doing a simple blood test without the necessity of the risky invasive ones on the expectant mother. NIPT can be performed as early as the 9th week of pregnancy, therefore giving the expectant mother more time for any follow up diagnostic prenatal genetic testing procedures and/or point of care counseling regarding the pregnancy.
One does not need any special procedures during non-invasive prenatal genetic testing. A phlebotomist simply needs to take some blood from the vein of the expectant mother. Then the geneticist isolates the DNA from the specimen and undertakes rigorous screening for genetic diseases or genetic conditions. In general, this non-invasive prenatal genetic screening is used for the following purposes:
- Determination of the sex of the fetus;
- Determination of trisomy (complete extra set of chromosomes, for a total of 69 chromosomes. About 1 in 1000 pregnancies is affected);
- Determination of the absence or presence of an extra chromosome (aneuploidies);
- Determination of the number of sex chromosomes;
- Determination of microdeletion syndromes (a small piece of the chromosome is missing);
- Determine if single gene mutations are associated with certain common genetic disorders in the population.
Non-invasive prenatal genetic testing, when used as a screen, has many advantages when other prenatal testing methods fail to detect a genetic disease or genetic disorder in the developing fetus during the pregnancy. For instance, a lack of family history does not necessarily omit a fetus from having a genetic disorder since some often could arise de-novo. Fetal ultrasound findings are sometimes not always a reliable indicator to confirm or omit a genetic anomaly in the developing fetus during the pregnancy.
Unlike the diagnostic invasive prenatal genetic testing, non-invasive prenatal genetic testing (NIPT) does not give a definitive answer. It happens because of several factors such as:
- The amount of cell free DNA that comes from the fetus is usually very low, so the test cannot be precise;
- The fetus may have mosaicism – in other words, the fetus has cells with different genetic makeup in its body, like colorful mosaics;
- The presence of multiple fetuses in the womb.
That is why the result of the non-invasive prenatal genetic testing (NIPT) is reported as a risk. When this risk is calculated, many factors are taken into account, such as:
- The age of the mother and father;
- The presence of genetic disorders in the family;
- The presence of a disorder in the parents;
- The results of the ultrasound scans and the biochemical tests.
Based on all of these data, the geneticist may conclude, for example, that there is 1 in 1000 or 1 in 200 chance of the fetus having a genetic disease or genetic disorder during early pregnancy. Though geneticists are very careful with the results of the non-invasive prenatal genetic test (NIPT), this test is very accurate. Its predictions for Down’s syndrome are correct in 99% of cases and 97-98% of cases for Patau’s and Edward’s syndrome (PMID: 26781507). Based on this chance, the expectant mother can decide whether or not to follow up with invasive prenatal genetic testing of fetal specimens obtained from either amniocentesis or chorionic villus sampling.
What is invasive prenatal genetic testing ?
Invasive prenatal genetic testing is characterized as being a diagnostic genetic testing of fetal samples that are invasively harvested from the expectant mother’s womb. It is also used to confirm the results obtained from non-invasive genetic testing or screening. The sources of such fetal samples are obtained from either procedures: chorionic villus sampling or amniocentesis. Since the quality and yield of fetal DNA obtained by these methods are very high, they make it feasible for a geneticist to use a large number of genetic laboratory techniques to confirm or refute the diagnosis of a wide assortment of genetic diseases or genetic conditions. However, depending on how successful the invasive sampling technique was applied, there could be a residual risk of having fetal genetic material contaminated with the expectant mother’s genetic material which could obscure the interpretation of diagnostic genetic testing findings. Furthermore, both chorionic villus sampling and amniocentesis can be risky. Since these fetal sample harvesting methods are invasive, they can trigger a miscarriage or cause an infection. Usually, there is about a 1% risk of miscarriage. Other complications can arise as well, such as excessive bleeding, cramping, or the labor can begin earlier than expected.
What is prenatal genetic testing using chorionic villus sampling (CVS) ?
In the womb, the fetus is attached to a structure called placenta which serves both as a protection and a source of necessary nutrients. The placenta contains special units called chorionic villi that look like tiny, thin fingers. It is possible to reach them with a needle through the expectant mother’s abdomen or with the catheter inserted through the cervix.
The placenta has the same genetic composition as the cells of the fetus. If one gets a sample of the villi cells, it is possible to look at them and evaluate the number and composition of chromosomes inside them. Chorionic villi sampling, or CVS can be performed starting from the 10th week of pregnancy. Undertaking prenatal genetic testing on such fetal specimen can be used for the diagnosis of most genetic disorders or genetic diseases, but not for conditions such as defects of the neural tube.
What is prenatal genetic testing using amniocentesis ?
In the womb, the fetus is surrounded by a fluid that protects it. It is possible to harvest a sample of this fluid by putting a needle through the expectant mother’s abdomen. This procedure is called amniocentesis. The cells of the amniotic fluid are called amniocytes which are derived from the fetus and are genetically identical to the fetus; and usually originate from the skin or other organs. Amniocentesis can be done no earlier than 14 week of the pregnancy. Undertaking prenatal genetic testing on such fetal specimen can be used for the diagnosis of most genetic disorders or genetic diseases, including defects of the neural tube.
To learn more about how prenatal genetic testing options can suit your needs, contact us at [email protected], and a representative will guide you through the testing process and provide more information for you and your healthcare provider. CEN4GEN is a DNA testing company / DNA testing organization / Genetic testing company / Genetic testing organization offering affordable DNA testing services / Genetic testing services in Edmonton, Alberta, Canada.
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