Preimplantation Genetics Screening – PGS – what is it? Does PGS influence IVF success rates?
What is PGS? Why is it used and why one may be advised to use it in IVF treatment? What does the embryo genetic testing look like? What is the influence on the whole IVF cycle and the whole process? What are the pros and cons of this procedure?
Our 3 experts will provide you #IVFANSWERS:
- Dr Maria Arque – International Medical Director in Fertty International
- Dr Victoria Walker – Fertility specialist – Institut Marquès
- Ester Padilla – Senior Embryologist – ProcreaTec
Answer from Dr Arquè - International Medical Director
Pre-Implantation Genetic Screening (or testing) is a procedure that checks whether or not an embryo is euploid – that is, whether it has the correct number of chromosomes. The results of this test tell us which embryos are the best candidates for a transfer. Only transferring embryos which we know are chromosomally normal gives us higher chances of success.
On its own, PGS does not improve pregnancy chances during a cycle, as pregnancy depends on more factors than just embryo quality. However, pregnancy rates are higher and the likelihood of miscarriage is lower when euploid embryos are transferred, as genetically imperfect embryos are one of the most common causes of pregnancy loss. The time to pregnancy also decreases through PGS testing, as we’re eliminating the possibility of transferring aneuploid embryos.
Answer from Ester Padilla - Senior Embryologist
PGS is the old name for the technique now called PGT-A (Preimplantation Genetic Testing for Aneuploidies). It can increase IVF success rates, especially in patients with a history of failed treatments. It is not a way of improving embryo quality, however. All it allows us to do is to discard those embryos which have chromosomal abnormalities – as a result, we only transfer healthy embryos. Because of this, we reduce the number of transfers, the chance of miscarriage, the time to pregnancy, and costs for IVF patients, while simultaneously increasing the chances of a healthy pregnancy.
We evaluate developing embryos by checking several different parameters. The quality of the embryo is initially judged only by a morphological assessment, not a genetic one; only PGT, however, can give us the full picture of an embryo’s health.
There is an established link between a woman’s age and the ratio of aneuploid embryos her eggs result in; this, in turn, leads to a decline in overall fertility. Even in egg donors, up to 35% of generated blastocysts could carry genetic defects. The reason for that is cell division – that is, in women of an advanced maternal age, eggs are less likely to undergo correct division, leading to cells with an incorrect distribution of chromosomes.
Embryos generated from oocytes of women in an advanced maternal age are more likely to have chromosomal abnormalities, even if morphologically they look healthy; as such, the only way to detect truly healthy embryos is PGT.
IVF failure can also be caused by the male factor. Although problems with the male factor usually cause problems such as fertilisation failure or the arrest of embryo development, the sperm can also carry aneuploidies, structural chromosome abnormalities, meiotic defects, and others.
The bottom line is PGT can increase IVF success rates by helping fertility specialists only transfer genetically normal embryos which have a greater chance to result in a healthy pregnancy.
Answer from Dr Walker - Fertility specialist
Can PGS diagnosis increase the success rates in IVF? If you have two embryos – one that has been biopsied and is known to be genetically normal, and another, which has the same morphological quality (and we can tell it’s genetically normal even without a biopsy, due to the way it developed), then these two embryos have the same chance of implanting. Thus, the biopsy itself doesn’t increase the implantation chance, but it also doesn’t reduce it.
The important concept is whether the embryo looks normal when examined under a microscope; at the moment, we can’t tell whether or not it’s genetically normal simply by examining it visually. Women under 35 have a high chance of producing genetically normal embryos; that chance goes down with age. The probability of producing genetically abnormal embryos can also be increased with ovarian stimulation; that probability can also be profoundly affected by male factor infertility.
PGS identifies the genetically normal embryo; in that way, it has been shown to change three things: it can decrease the number of embryo transfers needed to create an ongoing pregnancy, decrease the time needed to establish a healthy pregnancy, and it can also reduce the risk of miscarriage.
To answer the question, does PGS increase the success rates in IVF, the answer is both yes and no. For an individual patient who requires IVF, PGS can decrease the time it takes to achieve pregnancy, even if the biopsy itself doesn’t increase the chances of the embryo implanting compared to one that hasn’t been biopsied.