• Dr Peter Chew

Prenatal genetic screening tests

Most expectant women would like to know whether their unborn babies have any genetic abnormalities. Now, there are screening tests available to evaluate such risk. These tests, usually done between the late first trimester and early second trimester of pregnancy, involve measuring the level of certain substances in the maternal blood and doing an ultrasound scan of the baby. The data are then used to calculate the risk.

Common screening tests are as follows: NIPT NIPT or Non-Invasive Prenatal Test: This is the latest and most accurate screening test. During pregnancy, maternal blood contains genetic material (DNA) from the foetus. These cell-free DNA fragments can be analysed for chromosomal disorders. The test is done by taking maternal blood at about 10 weeks of pregnancy and screened for common foetal chromosome anomalies such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) ,trisomy 13 (Patau syndrome ) and sex chromosome abnormality. The accuracy for screening for Down’s syndrome is 99 %.

OSCAR test OSCAR stands for "One-Stop Clinic for Assessment of Risk". It is a screening test for Down's syndromes and is done during 11th –13th weeks of gestation. It consists of the measurements of nuchal translucency of the foetus (NT) and maternal serum level of Pregnancy Associated Plasma Protein-A (PAPP-A), and free beta-human Chorionic Gonadotropin (free beta-hCG). The risk is then computed based on the maternal age and the gestational age of the foetus. This test has an accuracy rate of approximately 92%.

Triple test Blood is taken from the mother between 14 to 18 weeks of gestation and three hormones --alpha-fetoprotein(AFP), human chorionic gonadotrophin (HCG) and oestriol –are measured .A risk value is calculated according to maternal age and the gestational age of the foetus. As its accuracy rate is around 70%, this test is less commonly done nowadays.

Nuchal Translucency (NT) The NT test involves a detailed ultrasound scan of the baby at 11- 14 weeks of the gestation .The thickness of the fluid collection behind the baby’s neck is then measured. A thickened NT of 3.5mm or more is associated with chromosomal abnormalities The accuracy of the NT test alone is about 80%.

The screening tests pose no risk to the mother and the baby. But they are not diagnostic. They only assess the degree of risk the baby may have and do not indicate that the baby actually has the problem. They can also give false positive results. That means they can be wrong and could mislead the expectant parents to believe their unborn babies might have genetic abnormalities. The only way to know the truth is to do the invasive diagnostic tests such as amniocentesis or chorionic villus sampling (CVS), which carry the risk of foetal injuries and miscarriage. (REFER Down’s syndrome article)

The decision to go through genetic screening tests would depend entirely on the expectant mother and her spouse as a positive result could lead to anxiety, stress and conflicting emotions. The couple should think through carefully whether to carry on further diagnostic tests should the result be positive.

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