A screening test is carried out for a large group of low risk patients to find out if they are at high risk for a particular disease. It is not meant for those who have already been placed in the high risk category based on the existing factors such as age or racial risk group.
In obstetrics, the most common screening test for chromosomal abnormalities, especially Down’s Syndrome, is the Non Invasive Pregnancy Test.
It is carried out after 10 weeks of pregnancy and offered by many companies and labs.
Initially, it was an exclusive club of opportunity as the test used to be very expensive. But, as more samples are being sent to the lab, the cost is now more affordable. However, it is not a test for those in high risk groups.
A first trimester ultrasound screening test for soft markers between 11 to 14 weeks is also a screening test for chromosomal abnormalities. This is a diagnostic test for large structural abnormalities at this gestation period.
A diagnostic test is one that confirms if you have a disease or an issue. Examples in obstetrics for the diagnosis of structural abnormalities is a detail anomaly scan of the foetus between 18 and 22 weeks of pregnancy by an expert usually a maternal fetal medicine specialist.
During this test, the structure of the foetus is looked carefully – from the head, brain, face, chest, heart, limbs, abdomen, kidneys, bladder, spine, cord and placenta are looked into for structural abnormalities.
There will be some structures that will only show abnormalities later in the pregnancy. There are also some structures that function differently while the foetus is in the womb and will only change at birth, or shortly after that. When your doctor completes the examination, they would give you a report on the findings.
Another one is chromosomal abnormalities. The most common of these are Down’s Syndrome as these babies live until adulthood. Other common abnormalities include Potter’s Syndrome, Edward’s Syndrome and Turner’s Syndrome.
The two common diagnostic tests for these problems are Chorionic Villous Sampling (CVS) and amniocenthesis. CVS is usually carried out at 11 to 14 weeks of pregnancy, where the doctor will obtain a sample of the placental tissue through the abdomen or the vaginal route.
However, this test is rarely carried out due to technical problems and higher risk to the pregnancy.
The most common test for chromosomal abnormalities is amniocenthesis. It is advised for all high risk cases without doing any screening tests. It is a test carried out after 15 weeks of pregnancy. The simple test includes inserting a long but thin amniotic needle under
the guidance of a continuous ultrasound examination.
A small amount of amniotic fluid is taken out and sent to the lab. The whole procedure is carried out in the clinic within minutes and with minimal risk to the pregnancy. The sample is then sent to the lab.
The most common analysis is karyotyping, where 23 sets of chromosome is examined. Any part of the chromosome that is less or more than 450–700 bph (depending on the lab) will be picked up.
Microarray has many thousands of probes. It will pick up an excess or lack of these components suggesting chromosomal abnormalities, including micro deletions of the genetic material.
The ultimate test is called the exome sequencing test to look for the protein coding regions of the gene. This will not only pick up chromosomal abnormalities, but also genetic diseases associated with an abnormal protein production that leads to abnormal functions in the body.
However, with each added test, the cost will be much higher and not many can afford it. The choice is made on how far the patients want to go to ensure their baby is normal.
This article is contributed by consultant obstetrician & gynaecologist, maternal fetal medicine Datuk Dr H. Krishna Kumar.
