Screening for Down Syndrome Home > Services We Offer
What is Down syndrome?
The term "Down syndrome" was first described in the late 19th century. It was not until 1965 that the underlying cause was attributed to a chromosomal abnormality. Instead of the normal 46 chromosomes or 23 pairs with one pair of sex chromosomes, these individuals have an extra chromosome in the 21st pair (giving a total of 47 chromosomes). Down syndrome is also called Trisomy 21. (see below)
Why test for Down syndrome?
This is the commonest chromosomal abnormality with an incidence of one in every 800 babies born. Down syndrome is associated with mental sub-normality and some physical disabilities. The life expectancy is usually shortened.
How high is my risk?
The risk of Down syndrome is increased with increasing age. For example, the risk of having a baby with Down syndrome is one in a thousand for a 30 year old compared with one in a hundred in a 40 year old (see table below). The risk is also increased if there is a history of Down syndrome in the family.
TABLE OF RISK
Age Mother (years) Risk of Trisomy 21 Risk of all chromosomal Abnormality
20 1 in 1667 1 in 526
22 1 in 1429 1 in 500
24 1 in 1250 1 in 476
26 1 in 1176 1 in 476
28 1 in 1053 1 in 435
30 1 in 952 1 in 384
32 1 in 769 1 in 323
34 1 in 500 1 in 238
35 1 in 385 1 in 192
36 1 in 294 1 in 156
37 1 in 227 1 in 127
38 1 in 175 1 in 102
40 1 in 106 1 in 66
41 1 in 81 1 in 53
42 1 in 64 1 in 42
43 1 in 50 1 in 33
45 1 in 30 1 in 21
46 1 in 23 1 in 16
47 1 in 18 1 in 13
48 1 in 14 1 in 10
49 1 in 11 1 in 8
Reference: Hook EB. Rates of chromosomal abnormalities at different maternal ages. Obstetrics and Gynecology, 1981;58:282-285 and Hook EB, Cross PK and Schreinemachers DM. Chromosome abnormality rates at amniocentesis and in liveborn infants. Journal of the American Medical Association, 1983;249:2034-2038.
What tests are currently available for Down syndrome?
Two types of tests are available; screening tests and diagnostic tests. Screening tests for Down syndrome include the biochemical tests (e.g. double, triple or quadruple tests) and ultrasound scan (11-13+6 weeks nuchal scan).

Diagnostic tests include amniocentesis and chorionic villous sampling. Since diagnostic tests are "invasive" (involves sampling with a needle or small cannula / tubing) there is a procedure-related risk of miscarriage between 0.5 to 1%. However, screening tests do not jeopardize the pregnancy as they are "non-invasive".
E.g. Double test, triple test, quadruple test
What biochemical tests are available?
The biochemical tests involve taking a blood sample from the mother to measure certain proteins or chemicals in the blood. The "triple test" measures the levels of αFP and two pregnancy hormones called unconjugated estriol (uE3) and free ß-human chorionic gonadotropin (ß-HCG). It is called the "double test" if two of the substances are measured and "quadruple test" if four substances are measured. These tests can detect between 65 to 77% Down syndrome if it is done between 15-18 weeks of pregnancy. In other words, they will detect two of three Down syndrome but will miss one out of three.

More recently, the first trimester screening tests have become increasing available for mothers in Malaysia. This involves taking a small amount of blood to measure the levels of two substances, free ß-HCG and pregnancy associated plasma protein-A (PAPP-A), which is normally found in all pregnant women. This blood test detects 68% of Down's syndrome. This test is usually done in combination with an early ultrasound examination, which measures the nuchal translucency (thin layer of fluid usually found at the back of the fetal neck) and the presence of the nasal bone between 11-13+6 weeks of pregnancy. The combination increases the detection to over 90% of Down's syndrome.
What about ultrasound scan?
Recent studies showed that ultrasound measurement of the nuchal translucency (an area of fluid beneath the skin of the fetal neck) and presence of the nasal bone will detect almost 90% of Down's syndrome. Since it is "non-invasive", there is no risk of miscarriage. Eleven to 13+6 weeks gestation is an ideal time to measure the nuchal translucency (NT) and look for the presence of the nasal bone. An increased nuchal measurement and absence of the nasal bone increase the risk of Down's syndrome. If this is the case, a cvs or amniocentesis is advised to check the baby's chromosome.

More recent studies have combined ultrasound with first trimester biochemical markers to give up to 95% sensitivity or detection rate of Down's syndrome.
Tests Timing (weeks) for tests Sensitivity (Detection Rate %)
Nuchal translucency (NT) 11-13+6 83
NT+ Nasal Bone (NB) 11-13+6 90
NT+NB+ 1st trimester biochemistry 11-13+6 95
Triple Test 16-20 67
Double test 16-20 60
Table showing the screening tests that are available for Down syndrome screening and their detection rates.
What happens if the screening test is positive?
If the result of the biochemical test is positive, an ultrasound examination is usually advised. This confirms the gestational age of the fetus. If there is an increased risk of Down syndrome, a diagnostic test such as an amniocentesis or a chorionic villous sampling (cvs) is offered. Similarly, an increased risk of Down syndrome with an increased nuchal translucency and absence of the nasal bone warrants a diagnostic test.

A negative result or a negative screen is very reassuring, although it does not mean it is impossible for the baby to have Down syndrome. Typically, in addition to indicating if the test is positive or negative, the laboratory report will give more specific information about the risk such as "1:1500 risk of Down syndrome".
Which test(s) should I opt for?
Today's women are faced with many choices in pregnancy. It is normal for any pregnant mother to be concerned if the baby they are carrying is normal. Most doctors will advise women above 35 to have an amniocentesis because the risk of having a Down syndrome child is about the same as the risk of miscarriage with amniocentesis. However, with advancing laboratory techniques and high resolution ultrasound scan, the detection of Down syndrome has improved tremendously allowing a close to 95% detection with a combination of tests. Therefore, many women are opting for the non-invasive tests for Down syndrome

Furthermore, different women view risks differently. A 30 year old, for example, may want an amniocentesis to know for certain that her baby she is carrying has no Down syndrome, although her risk is one in 1000. Doctors may recommend the triple test but that may not allay her anxiety. Yet another woman at the age of 40 may not wish to have an amniocentesis although her risk is considerably higher (1 in 66). She may view her 2% risk as "low", as the chance of a normal baby is 98%. Hence, that puts the odds very much in your favor.

The decision to have an antenatal test for Down syndrome is entirely an individual one. What is most important is that you make an informed decision with all the information that is available to you.