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Chromosomal Screening Tests

 

Chromosomal (genetic) abnormality screening

 

Chromosomal abnormalities like Down’s syndrome affect about 1 in every 700 babies. The risk of having a child with Down syndrome does increase with mother’s age (age 20 1:2000, age 30 1:890, age 40 1:90), but mothers of all ages and races can have a child with Down syndrome.

 

Current RANZCOG recommendations are that all pregnant women be offered a screening test for fetal chromosomal abnormalities, regardless of the woman’s age. Screening may be a non-invasive ultrasound scan (Nuchal Translucency scan) in combination with a maternal blood test; a Non-Invasive Prenatal Test (NIPT) that allows isolation of baby’s DNA from maternal blood; or an invasive diagnostic testing such as Chorionic Villus Sampling (CVS) or Amniocentesis, where a needle is passed through the mother’s abdomen, into the placenta (CVS) or the amniotic sac (Amniocentesis) to obtain fetal cells for testing.

 

Each of these types of testing has their own advantages and disadvantages, so it is extremely important that you discuss with your obstetrician what each test involves, what the test does and doesn’t tell us, and the implications of the possible test results. It is important that you understand the difference between a screening and a diagnostic test, and the different levels of accuracy of the different screening tests. There is currently no single test that will screen for or diagnose every potential chromosomal or genetic abnormality.

 

The most common approach for couples wishing to pursue screening would be to have the Combined First Trimester Screening (“nuchal scan” and blood test) between 11 and 14 weeks of pregnancy. If this returns a high risk result, then there is now available the NIPT (Non-Invasive Prenatal Test) which can be used to further refine the risk, prior to proceeding to invasive testing, which still remains the only diagnostic testing option.

 

While the NIPT does offer a higher degree of “accuracy” in screening for extra chromosomes 21, 13 and 18 (and can also be used to assess any sex chromosome problems and determine the sex of the baby) it does not allow for the assessment of other problems (such as heart problems) which may be picked up at the nuchal scan. At this stage, if couples were wishing to proceed with screening, I would not recommend seeing the NIPT as a replacement for the CFTS.

 

This can all be a bit confusing, so it is very important that you fully discuss your plans for screening with your doctor and understand why the tests are performed and what information they can provide.

 

Brief comparison table:

 

Test Gestation Accuracy Approx. Cost Comment
First trimester biochemistry screening (hCG & PAPPA) 10 to 13 60% $50  
Nuchal translucency scan 11 to 13+6 80 $200-300 Medicare rebate $60 if maternal age >35 or history of Down syndrome
Combined first trimester screening (CFTS) 11 to 13+6 85-90% $200-300 This involve a nuchal translucency scan at a ultrasound scan department and a maternal blood test at a laboratory.
Second trimester biochemistry screening (hCG, Oestriol, AFP, +/- inhibin A) 14 to 18 60-70%

(75% with inhibin A)

$65  
Second trimester ultrasound markers 18-20 20-30% $200-350 Low sensitivity and high false positive rate
NIPT (maternal blood test to obtain baby’s cell for testing) After 10 weeks 99% $400-600

 

There are 4 different providers. Please discuss with your obstetrician
Chorionic villus sampling (CVS) 10 to 12 99% $400 Invasive, risk of miscarriage 1%
Amniocentesis After 14 weeks 99% $400 Invasive, risk of miscarriage 0.5%

 

Please be aware that due to my own beliefs, I do not perform or refer for termination of pregnancy.

 

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