Antenatal screening tests can identify whether the fetus is more likely to have these certain conditions. These are minimally invasive tests designed to assess the risk of certain genetic disorders or birth defects in each individual pregnancy. These tests may help couples decide if they want to proceed with confirmatory invasive tests.
The most commonly offered is the OSCAR first trimester screening which involves a blood test for pregnancy-associated plasma protein-A (PAPPA) and human chorionic gonadotropin (HCG). This is combined with an ultrasound scan to measure the space at the back of the baby's neck (nuchal translucency) and is performed between 11 to 13 weeks and 6 days. This measurement and blood tests are combined with the mother’s age to calculate the risk of having a baby with Down, Edwards, and Patau syndromes. If the risk level is low, first trimester screening can be of much relief and offer reassurance of a healthy pregnancy.
There are other newer tests called Non-Invasive Prenatal Test NIPT (panorama test, harmony test for example) which are more sensitive and specific at screening for Down, Edwards, and Patau syndromes but they cost more. They can even screen for certain sex chromosome abnormalities, triploidy, microdeletions, DiGeorge syndrome, Angelman syndrome, Cri-du-chat syndrome, and Parder-Willi syndrome. The sex of the baby can also be determined. NIPT can be performed by taking a simple blood test on the mother from 9 weeks into her pregnancy as some of the fetal DNA crosses into the mother’s bloodstream. It takes 7-10 days for the results.
As the OSCAR and NIPT are all non-invasive screening tests, they can only tell us whether the baby is at high or low risk of having a chromosomal abnormality. Women who receive a high risk result will be offered confirmatory diagnostic invasive testing like amniocentesis or chorionic villus sampling (CVS).
Dr Christopher Ng
#04-03A Camden Medical Centre
1 Orchard Boulevard