WOW Health | Gynaecology
Antenatal Detection of DOWN SYNDROME
Down Syndrome is the most common genetic chromosomal disorder which causes learning disabilities and developmental delay in children. It also commonly causes other medical abnormalities, including heart and gastrointestinal disorders. It occurs in one in every 800 babies born.
It is caused when abnormal cell division results in an extra full or partial copy of chromosome 21. This extra genetic material causes the developmental changes and physical features of Down syndrome.
Anyone can have a baby with a chromosomal abnormality but the risk increases with the mother’s age. For example, the likelihood of carrying a baby with Down Syndrome increases from about one in 1,200 at age 25 to about one in 100 at age 40. But due to the higher fertility rate of younger women, 80% of children with Down Syndrome are born to women under 35 years of age.
There is no treatment for chromosomal disorder. If the baby is diagnosed as having Down Syndrome, one can then prepare for a baby with certain developmental problems or terminate the pregnancy. Diagnosis of Down Syndrome in a child is possible during first half of pregnancy when termination of pregnancy can be done if parents do not want to have a baby with this problem.
There are two types of tests for antenatal detection of Down Syndrome. First is called screening test which should be offered to all pregnant women in the first trimester and second trimester of pregnancy. Screening tests present the likelihood of a baby having this condition. If probability is low then no further tests are required. But if the probability is high, then a diagnostic or confirmatory test like Chorionic Villus Biopsy (chromosomal analysis of cells taken from placenta) or Amniocentesis (chromosomal analysis of cells taken from amniotic fluid) is done. These tests can tell whether the baby actually has this condition or not. While screening tests carry no risk for the mother or baby, diagnostic tests carry a small risk of miscarriage.
The screening tests include blood test and ultrasonography, ideally a combination of the two. Different tests have different detection rates.
First trimester screening
The first trimester blood test measures two proteins that are produced by the placenta: free beta-hCG and PAPP-A. A woman who is carrying a baby with Down Syndrome is more likely to have abnormal levels of these two proteins in her blood. The blood test can be done from nine weeks through the end of 13 weeks.
The NT test (ultrasound) measures the clear (translucent) space in the tissue at the back of baby’s neck. Babies with chromosomal abnormalities tend to accumulate more fluid at the back of their neck during the first trimester, causing this clear space to be larger. The NT screening can only be done between 11 weeks of pregnancy through the end of 13 weeks.
Together, these tests are known as the first-trimester combined screening and the detection rate of Down Syndrome baby is about 82 to 87 %
Second trimester screening
Second-trimester screening involves a blood test, commonly known as the multiple markers or the quadruple test. This test is usually done between 15 and 20 weeks of pregnancy.
The test measures the levels of four substances in blood: AFP, hCG, estriol, and inhibin A. Having abnormal levels of these substances in maternal blood is associated with an increased likelihood of carrying a baby with Down Syndrome. The test detects about 81 percent of babies with Down Syndrome.
Second-trimester screening is generally offered in conjunction with first trimester screening as part of what’s known as integrated or sequential screen
Integrated screening looks at the results of the first-trimester blood test, the NT measurement, and the second-trimester quadruple screen. It gives a single assessment of the risk of Down Syndrome after all of these screening tests are completed. Integrated screening detects 94 to 96 percent of Down Syndrome cases.
If calculated risk is more than one in 250, then only diagnostic test like Chorionic Villus Biopsy of Amniocentesis to confirm the diagnosis is done.
Another alternative is a blood test that can detect Down Syndrome and a few other chromosomal conditions at 10 weeks of pregnancy or later which has been introduced recently. It is called Non Invasive Prenatal Trisomy screening Test (NIPT). Any pregnant woman can have this but it is mainly offered to women who are at high risk of having a baby with a chromosomal disorder. The detection rate is almost 100%. But it still considered a screening test. That means that if the results indicate a problem, you still need CVS or Amniocentesis for a definitive diagnosis.
Good news for pregnant women in Nepal is that all screening tests for Down Syndrome can be done here. However, for the confirmatory test, one still has to go to India.
Dr. Veena Shrivastava is a Professor in the Department of Obstetrics and Gynaecology, Nepal Medical College and Gynaecologist in Nepal International Clinic. Send her your queries at firstname.lastname@example.org