Preimplantation genetic diagnosis (pre-implantation genetic diagnosis – PGD) is performed by removing one or two cells from the embryo in the laboratory when the embryo consists of 6–10 cells, that is on the 3rd day of embryonic development.

The genetic material of the cell is examined using molecular biology methods and analysed for the presence of gene mutations related to specific diseases known to be carried by the parents, meaning they are carriers of these conditions.

This technique is applied only to embryos created through in vitro fertilisation and before their transfer into the uterus, usually on the 5th day of development, at the blastocyst stage.

PGD helps couples who carry a known inherited condition (e.g., cystic fibrosis, beta-thalassaemia trait) and where there is a significant probability that embryos may be affected by the condition.

Embryos found to be positive for the condition during testing are not transferred to the uterus. This improves the success rates of the IVF attempt and also provides greater reassurance for the couple, knowing that only healthy embryos will be transferred.

 

Preimplantation genetic screening (chromosomal screening) of embryos (preimplantation genetic screening – PGS) is performed using the same technique and at the same stage of embryo development. However, in this case the analysis focuses on the chromosomes and the embryo’s karyotype, determining whether the embryo is genetically healthy and whether it has undergone any random chromosomal changes (which are not present in the parents).

If an embryo is found to have such chromosomal abnormalities (for example trisomy 21 – Down syndrome), it is not transferred to the uterus. This approach helps select healthy embryos and therefore improves the chances of a successful outcome.

 

Embryo biopsy can also be performed on the 5th day of embryo development, at the blastocyst stage. In this case, the embryo is more resilient to the small “micro-injury” caused by the biopsy, more cells can be collected for more reliable results, and embryos are not transferred immediately. Instead, the tested blastocysts are frozen (cryopreserved) and transferred in a later cycle, with only the healthy embryos being thawed and placed in the uterus.

In conclusion, preimplantation genetic testing techniques can help couples where the woman is of advanced reproductive age (>38 years) and therefore has a higher probability of chromosomal abnormalities, couples with a history of repeated IVF failures, couples who are both carriers of a hereditary disease, and couples who have been found to carry balanced chromosomal translocations during the investigation of recurrent miscarriages.