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Is there a grey area in prenatal CVS testing?

June 28, 2015

Huntington's Disease Youth Organization

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Is there a grey area in prenatal CVS testing?

Q. With prenatal CVS testing - is there a CAG count of the foetus where you DON’T have to terminate (eg between 36-39 the ‘grey area’ or 27-35)?

Sonnie, Young Adult, Australia

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A. Dear Sonnie,

Many thanks for your question. As you point out, direct prenatal testing for HD is only really offered on the basis that the couple would plan to terminate the pregnancy if it is found to have an expansion that would cause HD. This is because, if a pregnancy is continued following prenatal testing that shows an expansion, the status of the child would already be known from birth. This would potentially take away their right ‘not to know’, and goes against the internationally agreed guidelines for predictive testing for HD.

In most cases, prenatal testing gives a ‘clear cut’ result, i.e. a CAG count that is either in the normal range, or in the fully expanded range. However, you raise a very important question as to what the situation would be if the CAG count is found to be in the intermediate (‘grey area’) or reduced penetrance range.

For the intermediate range (27-35 repeats), we would not expect the child to be at any risk of developing HD themselves, and a termination of pregnancy would therefore not normally be offered on this basis. It would, however, be important to remember that although the child would not be at risk of HD, any children they may have could have an increased risk, particularly where the CAG repeat is closer to 35 repeats. For this reason parents should be made aware that genetic counselling could be offered to their child in adulthood to discuss their own options regarding pregnancy.
For the reduced penetrance range (36-39 CAG repeats), the question becomes more difficult. Some individuals with this size repeat will not develop symptoms of HD, and even if they do have symptoms, they may occur quite late in life. However, in terms of a prenatal result, the same issue would apply as for a full expansion - it is information that predicts (potential) future health problems, and therefore it would be preferable that an individual could decide for themselves when/if they want to know that information.

Because the decisions around prenatal testing are not straightforward (even with a ‘clear cut’ result), couples often find it helpful to discuss the options and potential scenarios in detail beforehand. A Genetic Counsellor would be able to give information about all the prenatal options available, as well as offering support around these decisions, and helping to plan for the different possible results. The likelihood of getting an intermediate/reduced penetrance prenatal result would also to some extent depend on the family history/CAG size in the parent, and a Genetic Counsellor could also assess this in more detail.

I hope this helps answers your question, but please do not hesitate to get back in touch if you have any other queries, or if you would like any help finding your local Genetics Service.