Before and during the pregnancy, parents-to-be undergo different genetic screening tests according to their ethnic descent, family history of genetic diseases, and abnormal test results that could indicate a risk for a genetic disorder.
Here is a short guide that will help you understand key terms and procedures related to prenatal testing.
Genetic tests during pregnancy – prenatal testing
Pre-pregnancy genetic screening – preconception genetic testing
Pre-pregnancy genetic screening tests (also known as preconception testing) detects carriers of genetic mutations. Normally, we all have two copies of each gene – one inherited from our mother and one from our father.
One normal copy is enough for each gene to function optimally. However, defects in both copies are reflected in a wide variety of ways, according to the defected gene.
Statistically, when both parents carry defected copies of the same gene, there is a 25% chance that their child will inherit two defected copies. This creates a high risk that can affect the baby’s health and even bring to untimely death. For this reason, many genetic disorders can be detected before the pregnancy, via a simple blood test.
What to do if both parents are diagnosed as carriers of defected genes?
When both parents are indeed found to be carriers of defected copies of the same gene during preconception genetic testing, they can choose to undergo a procedure called PGD (pre-implantation genetic diagnosis).
In this procedure, carried out during IVF (in vitro fertilization), healthy embryos are selected before implantation. This way, several embryos are tested for the defective gene copies, and after elimination only the embryos that do not carry the defected copies are selected.
Genetic testing during pregnancy
So far, we have only mentioned genetic testing performed before the pregnancy, the first type of testing is intended for parents, while the second type is for embryotic cells before implantation in the uterus.
We will now discuss a test that is performed during the pregnancy, when the fetus is already developing. This test is called amniocentesis, it includes drawing a small sample of amniotic fluid, which is the fluid surrounding the fetus. The test is recommended to women over the age of 35. It is designed to detect genetic disorders in the fetus, according to the tests already taken (nuchal translucency, maternal blood screening in the first trimester, triple test, inhibin test).
The test is usually performed between weeks 15-22 of pregnancy. During the test, around 20-30ml of amniotic fluid are extracted. Fetal cells are isolated from the amniotic fluid sample and analyzed in a laboratory to test for different genetic disorders.
When should you have genetic counselling / consultation?
Genetic counselling may be required in several situation. Parents can be referred to genetic counselling before birth when there is a known family history of genetic diseases. Other instances include consultation after a miscarriage due to a genetic disease, and if one of the parents is a carrier of a genetic disorder.
In addition, couples should have genetic counselling if they receive abnormal test results in blood tests or ultrasound tests. In certain cases, counsellors may discuss the option of pregnancy termination. In other cases, counsellors may refer the couple to further screenings and testing.
More diseases and tests
During the pregnancy, parents can choose to undergo further tests. In addition, genetic testing for newborns is offered after birth. Among the available tests during the pregnancy are maternal blood screening (a blood test that identifies chromosome abnormalities), alpha-fetoprotein test (between weeks 16-17 into the pregnancy), chorionic villus sampling (between weeks 11-13 into the pregnancy), and more. Common genetic disorders include Down syndrome, cystic fibrosis, Tay-Sachs disease and more.
