Prenatal Screening for Down Syndrome

Down Syndrome is a chromosomal disorder that results in a baby being born with an extra 21st chromosome, causing physical and mental developmental disabilities, as well as increased susceptibility to certain medical conditions.

Testing for Down Syndrome

In cases where either or both the mother and father have a family history of Down Syndrome, genetic counseling helps them understand the increased risks of chromosomal conditions in future pregnancies.

An obstetrician will also conduct a series of tests to detect symptoms of Down Syndrome before the child is born. Prenatal health screenings are available for all pregnant women to accurately gauge the chances of foetal abnormalities and genetic conditions. These include:

• One-Stop Clinic for Assessment of Risks of Fetal Anomalies (OSCAR):

  Conducted during the first trimester at around 11 to 14 weeks, this traditional test involves using a nuchal translucency ultrasound to measure the fluid at the back of the baby’s neck, whereby larger amounts of fluid signify foetal abnormalities. Blood tests are also done to check for irregular pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG) levels. Additionally, OSCAR takes into consideration the mother’s age, and performs a nasal bone assessment, as babies with Down Syndrome tend to have an absent or very small nasal bones.

• Maternal Serum Screening:

  During the second trimester at the 15th to 20th week of pregnancy, another round of blood test is done to detect biomarkers. Often, a quad screening test is done, in which alpha-fetoprotein (AFP), hCG, estriol and inhibin-A levels are measured for any abnormalities.

• Maternal Fetal DNA Blood Test (or Cell-Free DNA Test):

  With an accuracy of 99%, this is a highly sensitive test to detect genetic conditions such as Trisomy 21, 13 and 18 from the DNAs of both mother and baby. This test can be performed during the 10th week of pregnancy. The results usually become available after two weeks.

Those who’ve been found to have a higher risk of having a baby with chromosomal conditions during the prenatal screenings may be required to undergo further examinations. These diagnostic tests include:

• Amniocentesis:

  This process is typically carried out from the 15th week of pregnancy onwards. It involves inserting a needle into the amniotic cavity to get a sample of amniotic fluid for testing. The sample will then be assessed for abnormalities and extra chromosomes in the foetal cells. Amniocentesis carries a small risk of miscarriage, standing at around 0.5%.

• Chorionic Villus Sampling (CVS):

  This involves analysing chromosomal and genetically inherited abnormalities from the foetal cells of the placenta. It is carried out between the 10th to 13th weeks of pregnancy. Risk of miscarriage is higher than amniocentesis, standing at 1-2%.

• Cordocentesis:

  Carried out after the 18th week of pregnancy, this diagnostic test involves sampling the blood from the developing umbilical cord to check for chromosomal abnormalities in the cells. Risk of miscarriage is about 2%.

Down Syndrome is a random occurrence in chromosomal division, and is not caused by human error or environmental factors. However, it has been observed that mothers over the age of 35 and parents with a history of genetic conditions have a higher risk of having a child with Down Syndrome.

Obstetrician and gynaecologist Dr Kelly Loi from Health & Fertility Centre for Women offers a comprehensive range of services from pregnancy to childbirth, including prenatal health screenings and diagnostic tests for fetal and chromosomal abnormalities. Call (65) 6235 5066 to schedule an appointment today.