Advanced embryo selection
Why undergo genetic testing of embryos?
Each one of us has a genetic blueprint — an ‘instruction manual’ that makes us uniquely human and determines our individual characteristics, from the colour of our eyes to susceptibility to certain allergies. This blueprint is contained in every cell of our bodies and is packaged into structures called chromosomes. Normally, we have 46 chromosomes — 23 are inherited from the mother, and 23 are inherited from the father. Preimplantation genetic testing (PGT) allows us to determine whether an embryo has inherited the correct number, or arrangement, of chromosomes.
PGT involves taking a sample (or biopsy) of a very small number of cells from the outer layer of an embryo at day 5 or 6 of development. From this, we can make predictions about the chromosomes present in each cell of the embryo, allowing us to determine if it is normal (euploid) or abnormal (aneuploid). This helps decide on the best embryos to transfer, to reduce the incidence of failed transfers, miscarriage, or babies with chromosomal abnormalities.
At Flinders Fertility we offer PGT-A and PGT-SR, both of which use the latest technology, Next Generation Sequencing (NGS), to perform these tests.
PGT-A (Preimplantation Genetic Testing for Aneuploidy)
PGT-A (previously known as PGS - Preimplantation Genetic Screening), is a screening test used to determine whether there is an abnormal total number of chromosomes in an embryo. In most circumstances, an abnormal number of chromosomes is incompatible with life and if these abnormal embryos are transferred, they fail to implant successfully or miscarry. However, some abnormal embryos can carry through to birth and result in a baby with a chromosomal disorder — the most well-known example being Down Syndrome.
What are the benefits of PGT-A?
By screening out ‘aneuploid’ embryos and selecting the embryos that are tested as ‘euploid’, PGT-A screening can help select chromosomally normal embryos for transfer. Studies also show it may help reduce the average time to pregnancy and reduce the risk of miscarriage in an IVF cycle for patients over 40 years of age whose eggs have a higher likelihood of being abnormal due to the normal ageing process.
PGT-SR (Preimplantation Genetic Testing for known Structural Rearrangements)
PGT-SR can improve the chance of a healthy pregnancy in known carriers of chromosomal rearrangements and is performed alongside PGT-A testing. Chromosomal rearrangements refer to changes in the normal arrangement or size of a person’s chromosomes. People that have chromosomal rearrangements may not always show any effects as they may still have a ‘normal’ amount of genetic material. However, people with chromosomal rearrangements have an increased risk of creating an embryo with an incorrect amount of genetic material, which typically results in an unsuccessful pregnancy.
We recommend PGT-SR if you have had a child or pregnancy with a chromosome rearrangement, or if you or your partner are a carrier of a chromosomal disorder such as an inversion of translocation. Your fertility doctor will confirm with our testing laboratory that your specific translocation can be detected before going ahead with this test.
What are the limitations of PGT-A/PGT-SR?
At a first glance conducting genetic testing on every embryo may seem beneficial, however, it is important to remember that PGT only analyses a small number of cells and therefore cannot be considered 100% accurate. For this reason, even with a normal test result, we still recommend patients undergo antenatal screening. It is important to note too that PGT cannot change the chromosomal status of an embryo and therefore cannot increase implantation rates.
Risk of harm to the embryo
Embryos are fragile, so any procedure that involves handling the embryo and disturbing its surrounding conditions poses the risk of harm. However, while the risk of harm will always exist, it is thought that the overall risk of damage to the embryo is minimal. The level of risk is also related to how well the biopsy procedure is conducted. Flinders Fertility has experienced and skilled scientists to perform these procedures and they will let you know if they believe it is unsafe to proceed when they assess the embryo before biopsy.
When we conduct testing on a sample of cells from the embryo, we are assuming that this sample is representative of all the cells in the embryo. However, some samples show a condition called mosaicism, which is where some of the embryo’s cells have a mixture of abnormal and normal cell types. Embryos displaying mosaicism can sometimes still result in a healthy, live birth. To help avoid discarding these types of embryos, we also assess the level of mosaicism in the embryo. This gives patients the option of still proceeding with a transfer if the embryo only displays a low level of mosaicism.
The results of the genetic testing may return as ‘no result’, meaning the test could not determine whether the genetic material of the embryo was normal, or abnormal. This may occur for a variety of reasons, including a poor-quality embryo.
Your fertility doctor will discuss these limitations in detail with you and provide you with an information sheet. You may require or wish to have genetic counselling to discuss some of these limitations further. PGT-A/SR is also not suitable for patients who are at high risk of passing on a specific gene disorder (such as cystic fibrosis). This type of testing is known as PGT-M and it is currently not offered at our clinic.
When do I find out the results?
The embryo biopsy occurs within a normal IVF or ICSI stimulated cycle. After biopsy, the embryo(s) are frozen and the biopsied cells are sent to a laboratory (CooperGenomics) to be tested. Results are usually available in around 3 weeks. The clinical team will review the PGT results with you in order to decide which embryos are suitable for transfer in a frozen cycle. Abnormal embryos cannot be transferred and will be disposed of after you complete a consent form to do so.