In obstetrics, the most common screening test for chromosomal abnormalities, especially Down’s Syndrome, is the Non Invasive Pregnancy Test.
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 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.
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.
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.
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.
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.
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.
This article is contributed by consultant obstetrician & gynaecologist, maternal fetal medicine Datuk Dr H. Krishna Kumar.
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