Preimplantation Genetic Screening (PGS)
Flinders Fertility uses the latest technology, Next Generation Sequencing (NGS) to perform PGS testing.
PGS, also known as PGT-A (Preimplantation Genetic Testing for Aneuploidy), screens for an abnormality in the total number of chromosomes 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. 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, errors in chromosome segregation can sometimes occur. This mistake in cell division can produce abnormal cells with an incorrect number of chromosomes and the embryo may then contain both normal and abnormal cell lines. If both of these cell types are present in the embryo biopsy, the ‘mosaic’ status of the embryo will be detected by PGS. 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.
What are the benefits of PGS?
The purpose of PGS is to identify ‘euploid’ embryos with the correct number of chromosomes for transfer. By transferring embryos detected as euploid, PGS 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
What are the limitations of PGS?
PGS 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, however, a major consideration is the phenomenon of mosaicism where the results from a biopsy may not always be representative of the rest of the embryo. Furthermore, the possibility of poor DNA amplification from the biopsied cells means a conclusive result is not always achieved.
DNA is housed in structures called chromosomes. PGS using NGS technology is designed to detect large differences in the amount of DNA at the whole chromosome level. Much smaller changes in chromosomes or mutations at the DNA/gene level will not be detected using this technique.
PGS during an IVF cycle
The embryo biopsy occurs within a normal IVF or ICSI stimulated cycle. Good quality embryos at the blastocyst stage have around 5 cells taken from the outer layer of the embryo. These biopsied cells are then sent for PGS testing. Embryos are frozen after biopsy because the results of the PGS test will not be available for a few weeks. The clinical team will then review the results of the PGS test 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.
Find Out More
We do appreciate that a website may not cover all your information requirements so that's why we offer a number of information options. If you want to find out more about PGS, either:
- Call on 81 555 333 to talk to one of our Fertility Specialists.
- Email us at firstname.lastname@example.org.
- Seek a referral to Flinders Fertility from your Doctor.
If you require the aid of an interpreter please let us know, as well as any specific regional dialect that you may require.