In each human body, there are 23 pairs of chromosomes. Down syndrome occurs when the baby is born with an extra copy of chromosome 21 in its own cells. Down syndrome is also known as trisomy 21.
There are two types of prenatal tests for Down Syndrome:
Screening tests: These tests are not definitive. But if your child is at risk of Down syndrome, they will be aware of your likely chance. Screening tests do not harm the mother or the baby.
Diagnostic tests: These types of tests are very accurate and their answers will be definite. Diagnostic tests are usually performed in women with high-risk infants and based on the results of screening tests. Diagnostic tests can increase the risk of abortion, so it is not usually recommended for all women.
If you wish, you can do these tests and there is no requirement. If a screening test indicates that the baby is at risk for Down syndrome, you do not have to go to a diagnostic test. You and your family must decide which method is right for you, and you can talk to a doctor, midwife or genetic counselor.
There are 4 types of screening tests for Down Syndrome:
1- First quarter screening (FTS)
Screening tests are conducted in the first trimester between 9 weeks and 13 weeks (plus 6 days) during pregnancy to calculate the risk of a number of genetic disorders, including Down syndrome. This method is safe for mother and baby. The results of the following two tests are evaluated by the processor:
A blood test that is done between weeks 9 and 12 examines the hormonal changes that can cause problems in the child's chromosomes.
Ultrasound scan or ultrasound, performed at 12-13 weeks of gestation, measures the thickness of the fluid located behind the neck of the baby, known as translucent cervical fluid (NT). This thickness is often greater in neonates with Down syndrome.
These results, together with the increase in maternal age, indicate a risk of Down syndrome. If the child is at risk, an optional diagnostic test for diagnosis and confirmation of Down syndrome is likely to be required in the child.
2- Non-invasive test before birth
Non-invasive pre-natal testing (or NIPT and sometimes Harmony or Percept) is a new screening form for Down Syndrome. Which includes a simple blood test and the DNA of the baby is taken through the blood flow through the mother and the fetus. This test is done after 10 weeks of pregnancy and its precision for Down syndrome is over 99%.
The NIPT test is performed only in private clinics and is not covered by health insurance. This type of test is best suited to women who are more prone to having babies with Down syndrome.
3- Quad marker test
Maternal blood is taken for the four common parts of the baby's blood, the brain, the spinal fluid, and amniotic fluid. This test is done between the 15th and 20th weeks of pregnancy. This screening test measures 4 specific maternal blood components including AFP, HCG, Sterile, and Inhibin-A.
4. Screening the mother's serum in the second quarter
The second trimester screening, sometimes referred to as MSS or Triple Trials, is performed between the 14th week of pregnancy. This test is usually recommended to women who did not perform screening tests in the first trimester, or this test was not performed in the relevant labs.
The test involves a blood test to find hormones that may indicate Down syndrome in the baby or a nerve-defect in the child. If your child is at risk, a diagnostic test is recommended that is optional and to ensure that Down syndrome is present in the fetus.
In addition to triple trials, most pregnant women undergo morphologic scans (also known as abnormal scans or anatomy) during the 19th to 20th week of pregnancy to examine the growth rate of the fetus and the progression of pregnancy. Sometimes symptoms of Down syndrome also show up at this stage, but not always.
Screening tests are recommended for women with the following conditions:
Family history of birth defects
Women aged over 35 and over
Use of dangerous drugs or taking medications during pregnancy
Having diabetes and using insulin
Infection with viral infections during pregnancy
Exposure to high levels of radiation
The only way to diagnose Down syndrome in the baby is to do diagnostic tests. Diagnostic tests increase the risk of abortion, so that it is usually recommended only for women at risk, women with pre-existing neonates with genetic disorders and women with a family history of a genetic disorder. Two diagnostic tests that can confirm Down syndrome:
Coronary Artery Disease (CVS) Sampling: A needle, under the guidance of an ultrasound probe, is taken from the mother's abdomen into a newborn's cavity and biopsies the cells in the pair. This specimen may represent inaccurate, abnormal, and abnormal chromosomes. This procedure is performed between 11 and 14 weeks of gestation. This method is not so painful and takes about 20 minutes. The risk of abortion is less than 1 in 100.
Amniocentesis: A needle underneath ultrasound probe is inserted into the abdominal cavity of the mother and sampling from the amniotic fluid. In this method, existing, additional or abnormal chromosomes are tested. This procedure is performed between 15 and 18 weeks of gestation. This method is not so painful and takes about 20 minutes. The risk of abortion is less than 1 in 100.