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Genetic testing in pregnancy

Let me say at the outset this is a difficult area- emotionally, ethically, and the complexity of the choices available. I will help you as much as I can digest what the current state of play is. Please feel free to get me to clarify any uncertainty when you see me in person.


Aneuploidy screening is the term applied to the detection of chromosomal abnormalities before birth. All of these tests are optional. The main condition being screened for is Down syndrome (an extra copy of the 21st chromosome) but other abnormalities can turn up as part of the process.

Down Syndrome (Trisomy 21) is a condition in which a baby carries an extra chromosome. Chromosomes contain the genetic blueprint that regulate growth and development. Babies with Down Syndrome have an extra chromosome and this creates various problems. Children with Down syndrome are at increased risk of  abnormalities of the heart, bowel, hearing, and breathing. They are also at increased risk of developing diabetes and dementia, and intellectual disability of varying degrees.


The degree of learning disability cannot be determined prior to birth.  It may be profound (10%), severe (70%), or mild (20%). About 45% of individuals with Down syndrome will survive to the age of 60.
The risk of having a baby with Down Syndrome increases with advancing maternal age but women at ANY age can conceive a baby with Down Syndrome. The approximate risk of having a baby with Down Syndrome is age dependent, set out in the table below;

                      Age     Risk Of Down Syndrome

                                   At Birth              At 12 weeks

                        20    1:1500                1:1000
                        25    1:1350                  1:900
                        30      1:900                  1:600
                        32      1:660                  1:440
                        34      1:440                  1:300
                        35      1:350                  1:240
                        36      1:280                  1:190
                        37      1:200                  1:150
                        38      1:170                  1:110
                        40      1:100                    1:65
                        42        1:55                    1:35
                        44        1:30                    1:20


The risk of having a baby with Down Syndrome at birth is less than the risk at 12 weeks. This is because a percentage of babies with Down Syndrome will not make it through the pregnancy. That is, in the absence of a termination, the mother would experience a miscarriage or stillbirth.

Screening may be performed (see below for the options). There would be out of pocket expenses, the amount depending on the test.  A few general comments about screening, and if you wish definitive testing;

  • There are lots of conditions (eg. Autism) where, at the moment, all screens  will be normal. That is, we cannot check for a "normal" baby, but we can check for some chromosome problems.

  • Screening will not pick up 100% of cases of Down Syndrome. That is, a screen may suggest that a baby does not have Down Syndrome even though the baby does. This is called a false negative finding. 

  • There is a small chance that the screen will suggest that your baby has Down syndrome even though it does not. This is called a false positive finding. The NT+ has about a 5% false positive rate, while the cfDNA rate is about 0.1% 

  • If you have a NT+ or cfDNA screen that suggests a chromosome problem, then a CVS or amniocentesis test will be offered to decide if the result was a true positive (baby has Down Syndrome) or false positive (baby has normal chromosomes).

  • CVS or amniocentesis test (performed at 13-16) weeks has a risk of causing a miscarriage of about 0.5% (1:200).

 Whether or not you have screening or testing depends, in the main, on whether you would consider termination if the baby was affected.


 What do I think?. As long as you are prepared to accept that the screens (NT or cfDNA) study have false positive and false negative findings, I am more than happy to order the screen. If the screen suggests your baby may be affected then, after discussion, I am happy to organise formal testing (or not) to see if the screen was correct or not.

A summary of the NT and NIPT screens is;

  • Nuchal Translucency (NT) +/- 'Early Morphology' scanning

    • It is zero risk.

    • An ultrasound at 12-14 weeks can be performed by certain specialised ultrasound practices and measures an area of the fluid at the babies neck called the Nuchal Translucency (NT).

    • At the same time an 'early morphology' scan be performed to pickup problems that are found at the 19 week morphology scan.

      • The ability to pick up problems will depend on the expertise of the ultrasound practice.

    • A thicker than normal NT in combination with an abnormal protein in your blood (PAPP-A) is associated with an increased risk of chromosome problems.

    • In combination with a blood test the NT+ scan can detect 80-90% of cases of Down Syndrome.

    • It returns a risk ratio.

    • The risk ratio can vary from 1:2 to about 1:20,000.

    • A positive NT+ study is considered to be either more than 1:1,000 or worse, or at a risk that you judge merits further testing.

    • If you have a positive NT+ study, then;

      • either a NIPT test (for moderate risk of 1:50 to 1:1,000)

      • or a CVS-Amniocentesis (worse than 1:50) will be offered.

    • The NT scan may show up changes that are not associated with Down syndrome (and will not show up on the NIPT test). If those cases, it may be suggested that a CVS be performed, and the fetal sample collected be subjected to a special type of analysis called CGH.

    • The out of pocket cost for an NT scan ranges from $200-$300

  • Cell free DNA (cfDNA), Fetal free DNA (ffDNA), or Non invasive prenatal testing (NIPT).

    • Zero risk.

    • During pregnancy some of the fetal DNA is released into the mothers circulation.

    • To say that the reliable detetion and analysis of this DNA is complex is a gross understatement.

    • On the one hand it consists of a simple maternal blood test (zero risk), on the other this is an emerging technology.

    • There are a number technologies at the moment;

    • These are changing technologies. Please click on each link above if you may proceed with cfDNA and want to know more about the technology. It involves a maternal blood test at about 10-13w. Takes one week to process. Generally returns either a positive result (about 1:3 chance of Down syndrome) or a negative result (depending on maternal age that ranges from 1:10,000 to 1:150,000).

    • The out of pocket cost for an NIPT test is about $450 (June 2016)

    • The main providor of NIPT testing is Harmony-Douglass Hanly.

NT +/- NIPT, NIPT only, or Early morphology and NIPT?

There are various positions as to whether to have one test, both tests, or a mix.

  • Firstly, if you would not consider termination under any circumstances, then perhaps having no testing is the option for you.

  • One view is that you have an NT done, and if that risk is (say) more than 1:1,000 then have an NIPT. If the NT shows some findings suspicious for other chromosome problems then have a CVS with CGH testing.

  • Alternatively have an NT scan and if there are no suspicious findings suggesting non Down syndrome abnormalities, then have an NIPT done for the extra reassurance it provides.

  • The third option is just have an NIPT alone, and accept that there are rare cases of abnormailities that will be missed by the NIPT but may have been suggested by the pathway NT->CVS->CGH.

If the tests cost $1 each, then I would suggest that do both. If you could only afford one test, then I would suggest the NIPT test.

CVS & Amniocentesis

CVS is an invasive procedure. It is performed at around 14w. There is a less than 1:200 risk of miscarriage (and more recent studies suggest it is much lower than that). Rarely needs an amniocentesis if fails. If a termination is decided upon, can be done in theatre.

An amniocentesis is also an invasive procedure. It is performed at 15w. There is also an approximate 1:200 risk of miscarriage.   If termination decided upon, usually requires a labour.

CVS and Amniocentesis are invasive procedures, which enable the chromosomes or genetic material of your baby to be examined. These tests have a small risk (about 1:200) of causing miscarriage and will therefore are usually only offered in pregnancies where there appears to be an increased risk of chromosome or genetic abnormalities.​

More information

This is a complex evolving area. What follows are some links (these positions will change over the short term!);

  •  Society of Materno-fetal medicine (MFM) specialists

  •   Pathologists view

  •   College of GP's views

  •   Position of the RANZCOG

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