Background

Preimplantation Genetic Testing for Aneuploidy (PGT-A) in IVF involves testing embryos to check whether they have the typical number of chromosomes.

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Chromosomes are packages of DNA in each cell in our body. Embryos typically have 23 pairs of chromosomes (46 in total) in each cell.

PGT-A is a screening test which aims to distinguish between embryos that have a low risk of having extra or missing chromosomes (euploid) and embryos that have a high risk of having extra or missing chromosomes (aneuploid).

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Aneuploid embryos may have a missing whole chromosome (monosomy), extra whole chromosome (trisomy), a missing or extra part of a chromosome (segmental aneuploidy), or an extra or missing set of chromosomes (atypical fertilisation). In some cases, the change may be predicted to be present in some but not all of the cells (mosaicism).

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Embryos with a high risk of having extra or missing chromosomes are expected to have a very low chance of pregnancy. Transfer of an embryo with a high risk of having extra or missing chromosomes may result in not being pregnant following an embryo transfer cycle, a pregnancy loss at different stages of pregnancy (biochemical pregnancy, miscarriage, or stillbirth), or an ongoing pregnancy or birth of a child with a chromosome condition.

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Embryos with extra or missing chromosomes are common. The percentage of embryos that are aneuploid increases with the age of the egg provider.

• Under the age of 35, 20-30% are aneuploid.

• Between 35 and 40, approximately 50% are aneuploid.

• Over the age of 40, over 80% are aneuploid.

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PGT-A requires taking 3-10 cells from the outer layer of the embryo – this is called an embryo biopsy. There is a small chance of damage to an embryo when performing an embryo biopsy but it is overall a very safe procedure. All embryos are frozen while waiting for the PGT-A test results. DNA from the embryo biopsy is analysed to determine whether they have the expected number of chromosomes. Results will usually take around 4 weeks to complete from the time the embryos are biopsied.

 

Types of PGT-A

Monash IVF offer two types of PGT-A for our patients. The type of PGT-A used depends on the availability of DNA samples from the egg and sperm provider.

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1. If DNA from the egg and sperm provider is available, we use a DNA fingerprinting method which analyses small variations in the DNA code to detect extra or missing chromosomes and determine if they came from the egg or sperm provider. Extra or missing chromosomes detected using this method are expected to be present in all of the cells of the embryo. It can also detect rare events such as a whole extra set of chromosomes or missing set of chromosomes from one of the egg or sperm provider (atypical fertilisation). There is a lower chance of detecting mosaicism using this method.

2. If DNA from the egg and sperm provider is not available, we use a DNA sequencing method which analyses the amount of DNA fragments from each chromosome. This method can detect extra or missing chromosomes in the embryo biopsy but is not able to determine whether they came from the sperm or the egg. This means that extra or missing chromosomes detected using this method cannot be confirmed to be present in all of the cells of the embryo. There is a higher chance of detecting mosaicism using this method. This method cannot detect rare events such as a whole extra set of chromosomes or missing set of chromosomes.

 

It is important to remember that regardless of the method used, PGT-A is a screening test and therefore there is the chance of a false positive or false negative result. In any pregnancy following the transfer of an embryo which has had PGT-A, we recommend routine prenatal screening with ultrasound and prenatal cell-free DNA testing (NIPT).

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PGT-A does not test for specific genetic conditions (PGT-M) or for chromosome rearrangements (PGT-SR). If you and/or your reproductive partner have a high chance of a specific genetic condition, you can see a clinical geneticist or genetic counsellor to discuss your options for embryo testing in more detail.

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Should I have PGT-A?

We recommend speaking with your fertility specialist about the role of PGT-A in your treatment cycle. The decision to utilise PGT-A is an individual one and takes into account the expected number of embryos from each cycle, the expected rate of aneuploidy, the value of avoiding embryo transfers with a low chance of success and miscarriage and the potential for harm from additional embryo procedures and possible false positive results. PGT-A cannot improve the cumulative live birth rate from an individual IVF cycle but may decrease the time to pregnancy, the chance of miscarriage or the overall likelihood of pregnancy by focussing embryo transfers on the embryos with the highest chance of an ongoing pregnancy.

 

Where can I go for more information?

If you would like more information about the potential benefits and disadvantages of using PGT-A as part of your treatment, we recommend speaking with your fertility specialist. The Evidence-Based IVF website from Melbourne University is another good resource which explains the evidence for and against PGT-A. If you require more information about genetic testing prior to IVF or additional testing on embryos for specific genetic conditions, the genetics team are available to help you understand the options available.

 

References

Evidence-based IVF: PGT-A (Preimplantation Genetic Testing for Aneuploidy) (PGT-A testing) | Evidence-based IVF