Obstetrics Obstetrics




Down Syndrome Screening

Many pregnant women wish to undergo antenatal testing for Down Syndrome – for reassurance that their unborn child does not have Down Syndrome, to allow the option of termination if it does, or to allow preparation for the birth of a baby with the condition. Screening for Down Syndrome is offered as a routine part of antenatal care. There are two test options: the One-stop Clinic for Assessment of Risk (OSCAR) test and the non-invasive prenatal test (NIPT).

The OSCAR test is a prenatal screening test for Down Syndrome (Trisomy 21), Edwards Syndrome (Trisomy 18) and Patau Syndrome (Trisomy 13). The screening test is performed between 11-14 weeks of pregnancy. It involves blood tests to measure the levels of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG) hormones, and an ultrasound scan examination to measure a specific area on the back of the baby's neck known as a nuchal translucency measurement. These data, together with the mother's age, can be used to estimate the risk of the mother having a baby with Down Syndrome. The detection rate can be up to 90%, with a 5% false positive rate for Down Syndrome.

The non-invasive prenatal test (NIPT) analyzes the DNA that comes from the placental cells and are circulating in a pregnant woman's blood. The DNA in the placental cells is usually identical to the DNA of the baby. Genetic information from the baby can be obtained by analyzing the DNA from the placenta. The NIPT can screen for Down Syndrome, Edwards Syndrome, Patau Syndrome, and extra or missing copies of the X or Y chromosomes (the sex chromosomes). It can also screen for additional chromosomal disorders that are caused by missing sections of a chromosome called microdeletions. It has a higher detection rate of >99% and a lower false positive rate of ≪1% as compared to the OSCAR test for Down Syndrome. Therefore, it has become the more preferred screening test nowadays.

In our clinic, we offer safeT21expressTM as the NIPT of choice. The test is provided by Xcelom Limited and is the only brand in Hong Kong with exclusive patent from the Chinese University of Hong Kong. This test can be done after 10 weeks of pregnancy and it involves taking the mother's blood.