Advanced embryo selection
Flinders Fertility uses the latest technology, Next Generation Sequencing (NGS), to perform PGT-A testing for genetic screening.
What is PGT-A?
PGT-A (previously known as PGS - Preimplantation Genetic Screening), screens for an abnormality in the total number of chromosomes (genetic material) in an embryo. The test is carried out on a small number of cells removed from the outer layer of an embryo at day 5 or 6 of development, which is referred to as a biopsy. These biopsied cells are then analysed for the number of chromosomes they contain.
How are the results interpreted?
A normal ‘euploid’ embryo cell contains 46 chromosomes (22 pairs of the numbered chromosomes and one pair of sex chromosomes either XX - female or XY - male).
When an embryo forms with cells that have extra or missing chromosomes (incorrect chromosome numbers), this is called ‘aneuploidy’. When aneuploidy is present in every cell of the embryo the mistake has come from either the sperm, or (more commonly) the egg. The incidence of aneuploidy increases as women get older due to factors related to egg formation. While most aneuploid embryos will not form a pregnancy, those that do may result in a miscarriage or a baby with a chromosome disorder such as trisomy 21 (Downs Syndrome).
Sometimes a mixture of cell types can be detected in an embryo. This is called ‘mosaicism’. As an embryo rapidly divides after fertilisation, a mistake in cell division can sometimes produce abnormal cells with an incorrect number of chromosomes, and the embryo may then contain both normal and abnormal cells. While mosaic embryos have a reduced implantation potential, they can still produce healthy pregnancies if the abnormal cells are outcompeted by the normal cells as the embryo develops. There is also data suggesting that mosaicism in certain chromosomes is more likely than others to result in poor outcomes. A review by a geneticist may be recommended in certain situations prior to an embryo transfer being planned.
What are the benefits of PGT-A?
The purpose of PGT-A is to identify ‘euploid’ embryos with the correct number of chromosomes for transfer. By transferring embryos tested as euploid, PGT-A may:
- Increase the implantation rate.
- Reduce the average time to pregnancy in an IVF cycle.
- Overcome the adverse effect of age on an IVF cycle.
- Achieve pregnancy after repeated failed embryo transfers or miscarriages.
Can PGT-A detect errors caused by a chromosome translocation in the parent?
Yes, in many cases we will be able to perform Preimplantation Genetic Screening for known Structural Rearrangements (PGT-SR) alongside PGT-A. Atypical chromosome arrangement (or chromosome translocation) in a person can produce embryos with too much or too little DNA, which can often lead to failed implantation or miscarriage.
Although this test can identify embryos with the correct amount of DNA, it cannot differentiate between embryos that have inherited a balanced translocation (like the parent) and ‘normal’ embryos with typically structured chromosomes. Prior to beginning treatment, your fertility doctor will confirm with the testing laboratory that unbalanced inheritance of your specific translocation can be reliably detected.
What are the limitations of PGT-A?
It is important to note that there are certain limitations with these tests, which your fertility doctor will discuss in detail with you and provide you with an information sheet. You may require genetic counselling to discuss this further.
PGT-A is a screening test, which analyses a small number of cells and cannot be considered 100% accurate. This is due to a combination of technical and biological factors, as well as the occurrence of mosaicism. Furthermore, the possibility of poor DNA amplification from the biopsied cells means a conclusive result is not always achieved.
PGT-A is also not suitable for patients who are at high risk of passing on a specific single gene disorder. This type of testing is known as PGT-M and it is currently not offered by 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 away to an Adelaide-based laboratory (Perkin Elmer) to be tested, which can take a few 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 will remain in storage until the decision to dispose has been made by you, and the appropriate disposal forms have been signed.