Genetic Down Syndrome Testing: A guide

Genetic testing for Down syndrome may take different forms, depending on why and when the testing is conducted. Generally, the majority of genetic Down syndrome testing takes place prenatally, before an infant is born. 

What is Down syndrome?

It is a genetic syndrome caused by the presence of an extra chromosome 21. The disease is characterized by developmental delay, cognitive impairment on some level, as well as unique facial features characteristic of the syndrome. 

Down syndrome is estimated to occur in around 1 in every 1,000 births. The risk for Down syndrome increases with maternal age. 

The causes

95% of Down syndrome cases are identified as Trisomy 21. This is when an infant has an extra chromosome 21, meaning 3 instead of 2. When chromosome 21 fails so split during the reproductive process, it causes Trisomy 21. Usually this division failure comes from the maternal cells. 

Translocation and mosaicism are much rarer forms of the syndrome, with slightly different causes. 

Testing for Down syndrome?

Prenatal testing

There are several types of prenatal testing for Down syndrome which are performed during a pregnancy. Some of these are a standard part of prenatal care for all pregnant women, in the developed world, while others are recommended if a higher risk for Down syndrome is identified in either the mother or child. Older maternal age is currently identified as a risk factor for this genetic disorder syndrome, and after the age of 35 pregnant women are often referred to more extensive and accurate screening options.

Sequential Integrated Screening

This genetic Down syndrome testing is usually a standardized part of prenatal care in the US, although this may vary from state to state. This type of testing involves two stages.

The first stage takes place between the 10th and 14th weeks of pregnancy, and involves measuring the amount of fluid at the back of the developing baby’s neck (nuchal translucency measurement). Then a blood sample is taken from the pregnant Mother. The results from this, plus the fluid measurement are combined with the age of the Mother to assess the risk for Down syndrome. 

The second stage of the testing happens between the 15th and 20th weeks of pregnancy. Another blood sample is taken from the pregnancy mother, and combined with the results from the first stage of the testing. This gives a more accurate picture of the risk for Down syndrome.

This type of testing identifies the risk for developing Down syndrome in a fetus, it does not conclusively diagnose Down syndrome. If the tests show a potential high risk for Down syndrome, so the mother will be referred for more accurate and diagnostic testing.


Amniocentesis is not used only to test for Down syndrome, but other rare diseases as well, including cystic fibrosis and sickle cell disease with targeted testing. As a more invasive form of testing, it has a high degree of accuracy when it comes to diagnosing Down syndrome. 

The procedure occurs between the 15th and 20th weeks of pregnancy. A doctor uses ultrasound to help them insert a needle through the abdomen of the pregnant Mother, and a small amount of amniotic fluid is then removed. This fluid then undergoes karyotype testing which can identify genetic syndromes caused by chromosomal anomalies, such as Down syndrome. 

As an invasive form of testing, there is a known, and very small risk of miscarriage following amniocentesis. This risk starts from anywhere from 1 in 100, to lower according to the doctor or medical clinic performing the procedure. 

Chorionic Villus Sampling (CVS)

This type of genetic down syndrome testing takes place between the 10th and 12th weeks of pregnancy, and so much earlier than amniocentesis.. The testing involves taking tissue from the placenta, generally a very small piece. This piece is taken, again with ultrasound guidance and a needle through the abdomen, it is also put through karyotype testing to identify chromosomal changes, such as that which causes Down syndrome. The risk of miscarriage with the procedure, is similar to that with amniocentesis. 


Ultrasound can also be used as a part of genetic Down syndrome testing, and while it is not able to conclusively diagnose Down syndrome, it is another tool in helping doctor’s help identify risk for Down syndrome. An ultrasound can reveal physical characteristics of  Down syndrome. This might include identifying Down syndrome’s facial features both before and after birth. 

Genetic Counseling

Before embarking on any genetic Down syndrome testing, parents-to-be, should consider accessing genetic counseling for Down syndrome services. Genetic counseling is an invaluable service for anyone undergoing any type of genetic testing. 

When it comes to prenatal genetic testing, genetic counseling can walk parents-to-be through the options for genetic testing, and help them understand if they are at higher risk for a child with a rare disease.

Genetic counseling during pregnancy explains the types of testing, what each entails, and what the results of each might say. It can help parents understand what steps they can take, if any, following a positive diagnosis of Down syndrome or other rare disease. It also helps them understand their level of risk for a child with Down syndrome.

Genetic counseling provides crucial emotional support, information, and awareness about genetic testing and diagnosis options for Down syndrome. It is an important part of the testing process and should not be missed. 




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