What is Down Syndrome? 

Down Syndrome is the most common cause of significant mental retardation and learning disability in children. 

It is caused by a change in chromosomal number (genetic makeup) in the egg before it is fertilised by the sperm (at the time of conception). This usually occurs due to chance, and is more common in older mothers. 

As a result, the fertilised embryo contains an extra chromosome 21 making it three instead of the usual pair (hence the name trisomy 21).

It is important to know that people with Down Syndrome can also have reasonably long and fulfilling lives. 

Throughout the world, the frequency of Down Syndrome is about one per 700 births. 

The risk of having a baby with Down Syndrome increases as the mother’s age increases (refer to Table 11.1). 

How Do I Know if My Baby has Down Syndrome? 

Until recently, the only factor used to identify women at high risk for Down Syndrome was their age. 

At age 35, for example, the chance of having a baby with Down Syndrome is about one in 250. This has led to many hospitals offering amniocentesis to women over a certain age, usually 35 or 37. 

There are several other methods available to pick up Down Syndrome. These tests are grouped into screening and diagnostic tests.  

Screening Tests (FTS or MSS) 

Screening tests do not tell you if your baby has Down Syndrome. Their purpose is to tell you if your foetus belongs to a low or high risk group. 

If the screening test shows that there is a high risk of your baby being affected, you will be offered a diagnostic test (see below) to confirm it. 

Screening tests are noninvasive; hence, there is no risk of miscarriage to the baby. 

  • First trimester screening (FTS) — This consists of a detailed ultrasound scan of your baby at 11–14 weeks gestation to measure the nuchal translucency (NT). NT is the skin at the back of your baby’s neck (Figure 11.2). If this is increased above the normal range, most babies will still be normal although there is an increased risk of Down Syndrome, heart problem or rare genetic syndrome in some babies. Its accuracy is about 80 percent, and increases to 90 percent​ if maternal blood tests are done as well. This is known as integrated screening. 

A result of one in 300 means that 299 out of 300 women with this particular test result will not have an affected baby, and only one will. As you can see, it is not a test for the presence of a Down Syndrome baby, but a way of comparing your chance of having one. So, a 40-year-old woman would be very reassured by a result of one in 800 and a 20-year-old woman may opt for amniocentesis if her result was one in 100 (Figure 11.3). 

  • Maternal serum screening (MSS) — This measures certain hormones in your blood to determine your risk. These hormones are called alpha-fetoprotein, human chorionic gonadotropin, with or without oestriol. Blood is taken at between 15 - 20 weeks gestation, and a risk value is calculated, individualised to your age. The result of the test is also expressed in terms of a risk assessment (e.g. one in 300). 
​Overall, about six to seven out of 10 Down Syndrome babies will be detected by the serum screening. However, there will still be some that are undetected and will be born to mothers who have had a “low-risk” result. 
​These screening tests do not guarantee that the baby will be healthy. It only helps to screen for Down Syndrome. If the test result is “low risk”, this means that the chance of having this condition is reduced.​


It must be clearly understood that the results of screening tests represent risks. Increased risk does not mean that the baby is affected and further diagnostic tests must be done. 

A low risk does not exclude the possibility of Down Syndrome or other abnormalities as the risk assessment does not detect all affected pregnancies. 

Diagnostic Tests (Amniocentesis or Chorionic Villus Sampling) 

These are tests that obtain cell samples from the baby and can tell you for sure if the baby is affected with Down Syndrome. It is only performed for high-risk cases detected on screening due to the inherent risks of miscarriage associated with these procedures. 

The purpose of diagnosing this condition is to allow the couple the various options of whether to continue with the pregnancy or have an abortion.

  • Amniocentesis — Down syndrome can be diagnosed early in pregnancy from 15 to 20 weeks of pregnancy by amniocentesis. This involves a very fine needle being passed into the womb, under ultrasound guidance, and sampling of the amniotic fluid (water bag) around the baby. It takes about 2–3 weeks for the results to be ready although rapid tests (PCR) can also be done within 3–5 days. Most women do not find it too uncomfortable and takes about 5–10 minutes as an outpatient procedure. There is a risk of 0.5 percent of a spontaneous miscarriage after the procedure, which usually happens within two weeks after the procedure.
  • Chorionic villus sampling (CVS) — Chorionic villus sampling is another option that is performed even earlier at about 12 weeks of pregnancy. Like amniocentesis, it is also done under ultrasound guidance. A needle is inserted into the placenta to withdraw the cells through the abdomen or cervix. It allows earlier diagnosis and therefore reduces the anxiety of waiting. The risk of a miscarriage is similar to that of an amniocentesis.  
  • Fetal blood sampling (FBS) — This is a test that involves the sampling of fetal blood from the umbilical cord. The risk of miscarriage is much higher at two to three percent and thus, FBS is rarely performed for the diagnosis of Down Syndrome. 


After the amniocentesis or CVS, it is important to return to the hospital immediately if you run a fever, experience any unusual lower abdominal pain, vaginal bleeding or leakage of fluid from the vagina.


Source: Dr TAN Thiam Chye, Dr TAN Kim Teng, Dr TAN Heng Hao, Dr TEE Chee Seng John, The New Art and Science of Pregnancy and Childbirth, World Scientific 2008.

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