Everything about Prenatal Genetic Testing in simple terms

Are you a mother-to-be? Then your greatest wish is to give birth to a healthy baby. You probably already know about the blood tests and scans that are done during pregnancy to ensure your health and that of your baby. But have you heard about a special type of test that can detect in advance whether your unborn baby has a genetic disease or a birth defect? ​​Today we are talking about this very important topic that many people have questions about, namely Prenatal Genetic Testing.

Simply put, what is this genetic test?

To understand this, let's first look at what genes and chromosomes are. Think of our body as a big building. Genes are the complete blueprint, or set of instructions, to build that building. Chromosomes are like big books that hold these genes in order. When a child grows, half of these "books" are inherited from their mother and the other half from their father.

So, sometimes there may be some shortcomings, mistakes, or variations in these instructions, or in these "books". That's when a child may have a genetic condition or birth defect. So, Prenatal Genetic Testing is the process of testing the child before birth, during pregnancy, to see if the child has any such problem.

There are two main types of tests: let's understand the difference

These genetic tests can be divided into two main categories. It is very important to understand the exact difference between the two.

1. Screening Tests: These are tests that measure risk.

2. Diagnostic Tests: These are tests that confirm the condition of the disease.

Think of it like a weather forecast. A screening test says, "There's a 70% chance it'll rain today." It only says there's a high chance it will rain, not that it will definitely rain. A diagnostic test is like confirming with certainty that "it's raining now."

This difference can be understood more clearly from the table below.

What are the most commonly used screening tests?

There are several types of screening tests. Your doctor will recommend the ones that are most suitable for you.

1. Genetic testing of parents (Carrier Screening)

This is a very important test. This is not done for the child, but for the mother and father. There are some genetic diseases, and even though we have the gene that causes that disease in our body, we do not show the symptoms of that disease. We are called a 'carrier' . Imagine that you are a carrier of a certain disease, and your husband is also a carrier of the same disease, even if you both do not have symptoms, there is a 25% risk that the child will be born with that disease. Thalassemia, a common disease in Sri Lanka, is a good example of this.

• This is done with a simple blood test.
• Usually, the mother is tested first. If the mother is found to be a carrier, the father is also tested.
• This test should be done once in a lifetime .

2. Tests to look for abnormalities in the baby's chromosomes

Every cell in our body has 23 pairs of chromosomes, for a total of 46. Sometimes, when a baby is conceived, the number of these chromosomes can change. For example, if there are three of chromosome 21 instead of two, it causes a condition called Down Syndrome . There are several tests that can be done to check the risk of these conditions.

• Cell-free fetal DNA screening (NIPT): This is a Sinhala word meaning 'Non-Invasive Prenatal Testing'. This is a very advanced technology. When you are pregnant, there are very small amounts of DNA fragments from your baby floating around in your blood. This test uses a simple blood sample taken from you to separate those fragments of your baby's DNA and check for the risk of common chromosomal abnormalities such as Down syndrome. This can be done after 10 weeks of pregnancy .
• Serum Screening: This is also a test done on the mother's blood. However, this does not look at the baby's DNA, but at the levels of certain proteins in the mother's blood. Based on these protein levels, the risk of the baby having a genetic disease is calculated. Quad Screen is an example of this type of test. These should be done at specific weeks during pregnancy.

3. Tests to check for any physical abnormalities in the child's body

These are often done through an ultrasound scan.

• Nuchal Translucency (NT) Scan: This is a special scan done between 11 and 14 weeks of pregnancy. It measures the thickness of a layer of fluid under the skin at the back of the baby's neck. If this thickness is higher than normal, it could be a sign of a chromosomal abnormality, such as Down syndrome, or a problem with the baby's heart.
• AFP Screening (Maternal Serum Screen): This is a blood test done between 15-22 weeks. If the level of a protein called AFP is elevated in the mother's blood, it may indicate a problem with the baby's spine (neural tube defects) or abdomen.
• Fetal Anatomy Scan (Anomaly Scan): This is a scan that many mothers are familiar with. In this major scan, performed between 18 and 20 weeks , the doctor carefully examines every organ of the baby, from head to toe, including the brain, heart, kidneys, spine, limbs, and face.

Diagnostic tests that confirm the disease

If the results of a screening test are abnormal, or if you have a high risk of having a child with a genetic disease (e.g., being over 35, family history), your doctor may recommend a diagnostic test to confirm the disease.

These tests are very accurate because they use a sample of the baby's own cells. However, they are not as simple as screening tests. They are considered 'invasive' tests and have a very small risk of miscarriage (0.1% - 0.5%) .

There are two main types of diagnostic tests:

1. Amniocentesis: This is usually done between 16 and 20 weeks of pregnancy . In this test, the doctor, under the guidance of a scanner, inserts a very thin needle through your abdomen into your uterus and removes a small amount of the amniotic fluid that surrounds the baby. This fluid contains the baby's cells.

2. Chorionic Villus Sampling (CVS): This is usually done a little earlier, between 11 and 13 weeks of pregnancy . Here, a needle is inserted through the abdomen or vagina and a very small piece of tissue is taken from the placenta. The cells in the placenta are genetically identical to the baby's cells.

By sending these samples to a laboratory for testing, it can be determined with certainty whether the child has any chromosomal abnormalities.

Is it necessary to have these tests? Who is most important to them?

No, it is not mandatory to have these tests. This is a completely personal decision for you and your family. Before making that decision, you need to think about your beliefs, values, and future plans.

Some parents like to know about a medical condition before their child is born. That way, they have time to plan ahead, learn about it, and mentally prepare for the special care and medical treatment that the child will need.

Also, sometimes the results can be very disappointing, and some parents are forced to make very difficult decisions, such as whether or not to continue the pregnancy.

Typically, these tests are given more attention in the following situations:

• If a previous screening test result was 'high risk'.
• If someone in your or your husband's family has a genetic disease.
• If the mother is over 35 years old (because the risk of some genetic diseases increases with age).
• If you have had previous miscarriages or stillbirths.

Important questions to ask your doctor

Before making a decision about this, ask your doctor all the questions you have in mind and clarify them. Don't keep anything in mind.

• "Given my age and medical history, what screening tests are best for me?"
• "If the result of a screening test is abnormal, what do we do next?"
• "What are the risks to the baby or me if I have a diagnostic test?"
• "What is the probability of false positives in these tests?"
• "How long does it take to get results?"
• "Can a test like NIPT also determine the baby's gender?" (Yes, the NIPT test and possibly the Anomaly scan can also determine the baby's gender.)

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