PGS (Aneuploidy Screening)

 

Preimplantation genetic screening (PGS) is used very early on in the fertilization process. The aim of PGS is to increase the chance of pregnancy and reduce the risk of miscarriage for a specific subset of patients. These patients are usually older women who have an increased risk of miscarriages and those who have experiences recurrent IVF failures or unexplained recurrent miscarriages. Patients in these groups have been shown to be at an increased risk of producing embryos with an abnormal number of chromosomes, this is referred to as aneuploidy embryos.

New techniques enable us to examine all the chromosomes of an embryo from a few cell samples. The cells are removed (biopsied) from each embryo at the blastocyst stage. Blastocysts are embryos which have reached day 5 or 6 and are at a very immature, early developmental stage of the developmental process after fertilization of the eggs. A blastocyst is formed by 100 cells which are arranged in two layers, one layer in the inside and one layer on the outside. The inner cell mass will eventually form the baby and the outer cell layer (called the trophectoderm) develops into the placenta. In PGS 5 to 10

cells are be removed from the trophectoderm area. In most cases we can test the embryos, receives the results and transfer the embryos into the uterus all in the same cycle, however, in certain cases the results can take longer to analyse and the embryos are frozen via a process called ‘vitrification’. Once the test results are known, we can commence preparation of a frozen embryo transfer. The embryo biopsy procedure has been shown to not cause any harm to the embryo especially if the procedure is performed by an experienced embryologist.

If a couple chooses to do PGS they will have to go through a routine intracytoplasmic sperm injection (ICSI) procedure in order to generate several embryos that we can test. This is required in order to increase the chances of obtaining normal embryos. Because ICSI is a very precise method of inserting the sperm directly into the egg, this limits the chance of contamination of sperm around the resulting embryos which can lead to a misdiagnosis during the testing period.

 

What are the risks of PGS?

If there is not a sufficient number of egg retrieved after ICSI or if only a few eggs become fertilized, there is a high risk that no blastocysts will develop. In such cases, the PGS treatment cycle will be terminated.

In certain cases, after the biopsy of the blastocyst cells and subsequent diagnosis, sometimes all the embryos are found to have chromosome abnormalities, in which case there will be no embryo transfer. Despite PGS being performed, there is no guarantee of prevention of a miscarriage or of a live birth.

We know that as women get older, there is more chance that they will have a child with a chromosomally abnormality, such as Down’s syndrome. These women also have an increased risk of miscarriage, which is also due to chromosome abnormalities. Studies have shown that preimplantation embryos from routine ICSI patients frequently have abnormal chromosomes in a proportion of cells of an embryo, this is known as chromosomal mosaicism i.e. the chromosomes of the embryo may not be the same in every cell. This means that the cell or cells we biopsy may not reflect the status of the whole embryo.

Additionally, some embryos may be found to contain additional or missing fragments of chromosomes and this can have an effect on the development of the foetus however, the exact impact it will have is difficult to interpret. If a test result shows any signs of these issues, they will be discussed with you in the clinic and if you require additional information, we will refer you to our genetic counselling specialist to discuss further and answer all of your questions. Please note, however, that it is Bedaya Hospital’s policy is not to transfer embryos that contain any kind of chromosome abnormality, since it is impossible to know how many cells are abnormal and what effect there may be on the baby. We understand that there is an emotional, physical and financial burden associated with assisted conception and PGS therefore counselling is offered to all our patients.