Screening for Down Syndrome

ALL YOU NEED TO KNOW ABOUT SCREENING FOR DOWN’S SYNDROME

Down’s syndrome is a genetic disorder. The condition occurs when there is an abnormal cell division resulting in either an extra partial or full copy of chromosome 21. Because of this extra genetic material, there are some physical features and development changes associated with the Down Syndrome.

The severity of this condition can vary among individuals. For some, it involves developmental delays while for some others, it results in intellectual disability. It is important to have an understanding of the syndrome and catch the condition early so that the quality of life of the individual can be improved. Here are some common features of the Down ’s Syndrome:

• Small head
• Flat face
• Unusually shaped ears
• Short neck
• Short height
• Palpebral Fissures (upward slanting eyelids)
• Protruding tongue
• Excessive Flexibility
• Poor muscle tone
• Small hands and feet
• Short fingers
• Single crease in the palm
• Brushfield’s spots (small, white spots on the iris)

This is not a hereditary condition. It is caused by chance at conception, and can affect anyone. Also, since the extra chromosome responsible for the condition cannot be removed from the cell, this makes this disease incurable.

Should I Undergo A Test For Down’s Syndrome?

No matter what your age is while being pregnant, there will always be a risk of giving birth to a baby suffering from Down’s syndrome. However, this risk increases as the age of the mother increases. All pregnant women are advised to get a screening test. About 70% of all the babies born with Down’s syndrome had younger mothers. It is up to you to make the decision of finding out if your baby has this condition or not. Having the information regarding the tests can help you make the right decision.

WHAT TESTS ARE OFFERED?

For the Down’s syndrome, a screening test is conducted before the diagnostic test, as per the requirement. With the help of the screening tests, it can be determined if the babies have an increased risk of having the condition or any other chromosomal abnormalities. For the screening test, all the mothers go through the following two parts:

• Blood Test – This is a double marker test in which the amount of two chemical substances present in the blood of the mother is measured. This is performed between 10th to 13th weeks of pregnancy.
• Ultrasound scan – This is Nuchal Translucency (NT) scan which is performed between the 11th to 19th week of pregnancy.

After both the tests are completed, the results from both are combined with the age to estimate the probability of the unborn baby being affected by the condition.

If you are past the 13th week of pregnancy and didn’t have the double marker test conducted in the first trimester, you can have the risk assessment done through a blood test. This is known as a Quadruple marker test which can be performed between the 15th and 21st week of pregnancy.

A diagnostic test will need to be conducted for getting a definitive result on whether the baby has Down’s syndrome or not. This test is mostly conducted only when there is an increased risk of a miscarriage or pregnancy loss in the screening test.

Can I undergo only the NT scan?

Yes, it is possible to just get the NT scan done. The scan alone is perfectly capable of detecting about 70% of the pregnancies with the condition. However, the results received from the combined results of the blood and the scan test provide a higher detection rate than the blood test or scan alone. It can identify 90% of the pregnancies with the condition.

What are the types of screening test results I can expect to receive?

“LOW RISK” or “SCREEN NEGATIVE” (The most common result)

If the result of the screening test was lower than the cut off value of 1:1250, it means a screen negative or low risk result. However, it doesn’t mean that there is no risk. It simply means that the probability of your baby having this condition is extremely low.  If there is no other medical reason, you won’t have to undergo a diagnostic test. If you still want to get a confirmation on the genetic makeup of the baby, you can still opt for the diagnostic test.

“HIGH RISK” or “SCREEN POSITIVE” (Less than 5% i.e., 1 in 20 women)

If the result of the screening test is more than the cut off value of 1:250, it means that there is a screen positive or high risk result. After this, you will either have to go through a Non-Invasive Prenatal Testing (NIPT) or a diagnostic test. However, it is your decision to have a diagnostic test or not. The diagnostic test won’t have a risk to your pregnancy but will leave some uncertainty until the baby is born. If you decide to go through the diagnostic test, make sure that you have all the information regarding the tests. You will get a definitive answer through the diagnostic tests but there will be a small risk of miscarriage.

If none of the above options suit you, you can try the early anomaly scan that is one between the 16th and 18th week of pregnancy. Or, you can go for the routine detailed anomaly scan that is performed between the 18th and 20th week of pregnancy.

Diagnostic tests for Down’s syndrome

For Down’s syndrome, there are two diagnostic tests available:

1. CVS (Chorionic Villus Sampling) – In this test, the cells from the placenta are taken and analyzed to check for the condition in the fetal chromosomes. In most cases, this diagnostic test is conducted between the 10th and 13th week of pregnancy, i.e. in the first trimester. Also, the risk of miscarriage or pregnancy loss from this test is very low.
2. Amniocentesis – In this test, amniotic fluid sample is taken that surrounds the fetus. A needle is used to draw out this fluid from the mother’s uterus. The sample is used to analyze the fetus’s chromosomes. This test is conducted after the 15th week of pregnancy, i.e. in the second trimester. With this test also there is a low risk of miscarriage.

If a couple has undergone in-vitro fertilization and they are at a higher risk of passing the genetic condition, preimplantation genetic diagnosis is performed so that the embryo can be tested for any genetic abnormalities.

What are the possible results from diagnostic tests?

There are only two possible results from a diagnostic test:

• Baby has the condition
• Baby doesn’t have the condition

Every year, about 3000 to 5000 babies are born with this condition. So, the probability of the latter is significantly more than that of the former.

What happens if my baby has Down’s syndrome?

If your baby has been diagnosed with Down’s syndrome, there are two ways a parent can go:

• Decide that they don’t want to continue the pregnancy and choose to abort. Discuss the health consequences of abortion with the doctor.
• Decide that they want to have a baby. If you have selected this, you need to make a plan and prepare yourself with the extra challenges that come with raising a child with Down’s syndrome.