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Down Syndrome in Depth

Testing for Down Syndrome

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Pregnant2Day previously gave an overview of what Down syndrome is, who is at risk, what prenatal tests are available and what to do when faced with a positive test result. (To read this overview, click here). In this article, we expound on the prenatal tests available and their associated risks.

Please bear in mind that our aim is merely to present you with facts that will help you navigate some of the emotionally charged decisions you have to make during your pregnancy. If the baby you are carrying is indeed diagnosed with Down syndrome, we encourage you to become as informed as possible and to discuss the matter with a medical practitioner you trust before making any decisions. It is also advisable to speak with other parents of Down syndrome children.

As discussed previously, there are two types of prenatal tests available to detect Down syndrome: screening tests and diagnostic tests. Screening tests, which are non-invasive, estimate the risk that a foetus has of having Down syndrome and various other genetic abnormalities. These results are factored in together with the mother’s age, with the risks increasing after the age of 35, and much more dramatically after the age of 45. Screening tests are not diagnostic and the one risk they do carry is that of false positives. Pregnancies identified as ‘high risk’ by screening tests become candidates for diagnostic testing.

A diagnostic test will tell whether the foetus actually has Down syndrome or several other conditions. Diagnostic tests involve sensitive procedures and therefore carry a risk to the foetus. The following table summarises the tests available, when they are commonly performed and their associated risks.

(Note, the most accurate method to determine the gestational age of a foetus, which contributes to risk factors and evaluation of screening tests, is through ultrasound not by the date of last menstruation.)

TESTING FOR DOWN SYNDROME

SCREENING TESTS

Nuchal translucency screening

  • When performed

  • Method

  • Associated Risk

11–14 weeks
An ultrasound is performed on the nuchal area of the foetus - a fold of skin at the back of the neck - to measure Nuchal Translucency. Babies with chromosomal abnormalities have a larger space there due to an accumulation of fluid.
This is a non-invasive test and therefore does not pose any risk. A positive result, combined with the mother’s age will determine only if your risk for Downs is high or low. Foetuses with an increased area of nuchal translucency are at risk for other birth defects in addition to Downs so a positive result will usually require further testing.

Triple screen

  • When performed

  • Method

  • Associated Risk

15–18 weeks
The mother’s blood is checked for the maternal serum alpha-feto protein (AFP, a foetal liver protein), and pregnancy hormones unconjugated estriol (uE3), and human chorionic gonadotropin (hCG).
This is a non-invasive test and therefore does not pose any risk. A positive result, combined with the mother’s age will determine only if your risk for Downs is high or low. Foetuses with an increased area of nuchal translucency are at risk for other birth defects in addition to Downs so a positive result will usually require further testing.

 

Quadruple screen

  • When performed

  • Method

  • Associated Risk

15–20 weeks 79%
Sometimes a marker called inhibin A is added to the triple screen, resulting in the quadruple screen, which is slightly more accurate. Inhibin A is a protein secreted by the ovary, and its level is increased in the blood of mothers of foetuses with Down syndrome.
See triple screen above

Expanded Alpha fetoprotein (AFP) screening

  • When performed

  • Method

  • Associated Risk

15–22 weeks
This is a simple blood test, recommended for all pregnant women. The test measures the alpha feto protein, produced by the fetus, and free beta hCG, produced by the placenta. The rate of detection for women under 35 is about 60 percent for Down syndrome, with the test also detecting the possibility of other genetic abnormalities. The detection rate is higher in women over 35.
There are no risks to mother or child. If the result is positive, it means you have an increased risk of a genetic abnormality. However, it does not diagnose birth defects, so your doctor will usually recommend further testing.

Should your screening test results place you at high risk, always bear in mind that due to the number of variables at play, these results cannot be 100% conclusive and should only form a basis for deciding whether or not to undergo more tests. For a true diagnosis, it is essential to examine the chromosomes of the foetus – this involves diagnostic testing which carries a greater risk to the foetus. Genetic counselling will often precede diagnostic testing. There are several reasons why you may opt for diagnostic testing. These include making the decision to carry the child to term, wishing to investigate potential medical interventions, planning for and preparing the family for a child with special needs and locating support groups and other resources.

DIAGNOSTIC TESTS

Amniocentesis

  • When performed

  • Method

  • Associated Risk

15 – 18 weeks
Usually performed under local anaesthetic, this procedure is used to collect amniotic fluid, the liquid within the womb. A needle is inserted through the mother's abdominal wall, through the uterine wall, into the amniotic sac. Ultrasound is used to carefully guide the needle to an area as far away as possible from the foetus, where a small amount of fluid is taken for testing.The puncture does usually heal and the amniotic fluid is naturally replenished.
Although amniocentesis is routinely carried out, there are risks involved. These include infection of the amniotic fluid and continued fluid leakage from the puncture. The mother may also experience significant cramping. Worst-case scenario is an increased risk of miscarriage (with an increase of up to 1% above normal). There is also a minor risk of preterm labour and delivery, respiratory distress and postural irregularities.Amniocentesis is not recommended before the 14th week of pregnancy.

Chorionic Villus Sampling (CVS)

  • When performed

  • Method

  • Associated Risk

10-12/13 weeks
This test entails taking a small amount of tissue from the young placenta. This tissue is taken from what is known as the chorionic layer and is called the chorionic villus. Cells are collected in the same way as in amniocentesis but, depending on the mother’s anatomy, tissue can sometimes be extracted by inserting a tube into the uterus through the vagina. Cells within this placental tissue contain foetal chromosomes that can be tested for Down syndrome. The major advantage of CVS is that it can be carried out earlier than amniocentesis.
CVS carries the same risks as amniocentesis, with the risk of miscarriage after CVS increasing to 3 to 5% above normal. Studies have shown that the incidence of miscarriage is lessened with an experienced doctor performing the procedure. CVS before the 10th week of pregnancy has been linked with babies being born with missing or shortened fingers or toes. The risk of continued leaking of the amniotic fluid could result in the baby being born with underdeveloped lungs. It is vital for a mother who has undergone CVS to be monitored to ensure that no infection develops. Mothers considered high risk may have CVS, but the decision to use CVS as opposed to amniocentesis is an individual one, and should be thoroughly researched.

Percutaneous umbilical cord blood sampling (PUBS)

  • When performed

  • Method

  • Associated Risk

18 - 20 weeks or later
This test examines blood from the foetal umbilical cord to detect abnormalities. Ultrasound determines where the umbilical cord joins the placenta and then guides a thin needle through the abdomen and uterine walls into the umbilical cord to retrieve a minimal amount f foetal blood.PUBS is useful when conclusive data cannot be obtained through amniocentesis, CVS, or ultrasound.
This test carries an increased risk of miscarriage and other complications and is typically reserved for pregnancies determined to be at high risk for genetic defect.

For further informative reading about Down syndrome, visit the following sites or keep visiting www.pregnant2day.co.za for updates.

www.downsyndrome.org.za
www.downsed.org
www.ndss.org

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