What is Down's syndrome?

Down syndrome (DS) occurs when a baby is born with an extra chromosome 21, which is one of the most common chromosome abnormalities in humans, leading to intellectual disability and congenital heart disease.

About 1 in 800 babies is born with Down syndrome. A woman of any age can have a baby with Down syndrome. But a woman is more likely to have a baby with Down syndrome the older she gets. If the fetus is not diagnosed during prenatal testing, they will survive and be born.

Pregnancies at advanced maternal age(over 34 years old) are closely associated with DS, and the older the age, the greater the chance of having a Down's baby. The department of health's current policy is to recommend amniocentesis for fetal chromosome analysis by extraction from amniotic fluid in women over 34 years of age.

However, women of childbearing age under 34 years, after all, account for the majority. According to the statistics, 80% of Down's babies are born to women under the age of 34. For this reason, it is recommended that pregnant women under 34 be screened for Down syndrome.

What are the screening methods for Down syndrome?

There are currently two screening methods for the syndrome:

• The first trimester screening: Screening is performed at 11 to 13 weeks of pregnancy with ultrasound, which measures the nuchal translucency, as well as the development of the fetus' organs. Finally, pregnant women's blood is drawn to measure the values of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (HCG) in serum. Combined with the data of pregnant women's age, weight, race and gestational weeks, the risk value of having a fetus with Down syndrome is estimated, with the detection rate ranging between 85% and 89%.
• Second trimester maternal serum screening: Screening is performed by drawing blood from pregnant women at 15 to 20 weeks of pregnancy, which tests the alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG), unconjugated estriol (uE3), and inhibin A. Combined with the data of pregnant women's age, weight, race and gestational weeks, the risk value of having a fetus with Down syndrome is estimated, with the detection rate ranging between 80% and 83%.

Comparison of the two screening methods

After I get tested for Down's, can I be sure I won't have a Down's baby?

• This test is screening rather than diagnostic. If the risk is greater than 1/270, it is considered as high-risk, which does not mean that the fetus must have chromosomal abnormalities. It is recommended that pregnant women receive amniocentesis to confirm the diagnosis.
• The risk for Down's in the first trimester ranges from 1/271 to 1/1000, it is recommended that pregnant women receive second trimester maternal serum screening at 15 to 20 weeks of pregnancy. If the risk is greater than 1/270, amniocentesis will be performed.
• If the screening test is low risk, there is still a very small chance that your baby will have Down's syndrome or other chromosomal abnormalities, so this method does not eliminate all Down's births. Currently, the only way to confirm the diagnosis of chromosomal abnormalities is to undergo invasive tests such as chorionic villus sampling, umbilical cord blood sampling or amniocentesis

Arrangement for Down's screening

• The first trimester screening must be scheduled for ultrasound. If you have chosen this examination, you may inform your obstetrician to arrange the exam time, or dial (04) 22052121 ext. 2188 to make an appointment.
• The second trimester screening is carried out at 15 to 20 weeks of pregnancy. Ask the doctor for blood test.