Continuous Research and Development into DNA Molecular Diagnostic Methods
What are the diagnostic prenatal tests?
The diagnostic tests in the prenatal period are specialized tests such as amniocentesis, chorionic villous sampling (CVS) and foetal blood sampling (FBS) which are able to detect foetal abnormalities especially chromosomal abnormalities such as Down’s Syndrome. Where a genetic disorder is known, such as in Thalasemia (an inherited blood disorder), a gene probe for the defect is used to test for the condition in the baby.
Since these tests are “invasive”, i.e. they involve sampling with a needle or small cannula (tubing), there is a procedure-related risk of miscarriage between 0.5 and 1.0 per cent.
Amniocentesis
This is the commonest of the three specialized tests and involves drawing some amniotic fluid that surrounds the baby for analysis. This test is done under ultrasound guidance, using a long, narrow needle. It can be performed from 15 weeks onwards. The results usually take about 8-10 days to be ready. The risk of miscarriage is 0.5 per cent, over and above the normal risk of miscarriage. Amniocentesis is commonly performed to check for chromosomal abnormalities in women over the age of 35, those with positive triple test or when an abnormality is found on ultrasound scan.
Chorionic Villous Sampling (CVS)
Chorionic villous sampling is essentially a biopsy, where a piece of tissue is taken from the placenta. This can be done earlier, usually from 10-12 weeks of pregnancy. Placental tissue contains cells, which are similar genetically to the baby, is sent for analysis. Depending on where the placenta is situated, CVS can either be done through the trans-abdominal or trans-cervical (through the neck of the womb) route. It is associated with a slightly higher risk of miscarriage, between 0.5 and 1.0 per cent, and can be done earlier compared to an amniocentesis. Women with increased risk of chromosomal abnormalities (those above 35 years of age) and genetic diseases (such as ?- and ß-Thalassaemia) may be offered a CVS.
Based on Dr. Patrick Chia & Dr. S. Raman, Is My Pregnancy Normal?, Discern Publishing House, 2005.