PREIMPLANTATION GENETIC SCREENING (PGS)

PGS (Preimplantation genetic screening)

The aim of PGS is to identify structural or numerical chromosomal abnormalities (aneuploidies), including chromosomal deletions, duplications or translocations, in embryos from couples known to be euploid (chromosomally normal). Between 40 and 60% of preimplantation embryos are aneuploid, which may explain the relatively low implantation efficiency of both natural conception and IVF. The main indications for using PGS in IVF treatment include advanced maternal age, recurrent implantation failure and recurrent miscarriage. The success of PGS depends largely on technical skill, embryo quality and the presence of mosaicism in preimplantation embryos. The isolated cells are tested for structural or numerical abnormalities in a range of chromosomes using cytogenetic methods (fluorescence in situ hybridisation, FISH). Today, biopsy of day-3 embryos is the most common method. However, blastocyst biopsy is fast becoming the preferred method, because it reduces the likelihood of mosaicism and increases the amount of DNA available for diagnosis.

PGS is indicated for

• Couples where the woman is of advanced age. More than half of the embryos from women aged 35-40 have chromosomal abnormalities, and this proportion rises sharply as the woman's age increases.
• Couples with a history of recurrent miscarriage and/or failed IVF attempts.

The benefits of PGS are

• A higher chance of pregnancy
• A reduced risk of miscarriage
• Greater confidence in transferring a single embryo, avoiding the risks associated with twin or multiple pregnancy
• Fewer IVF cycles needed to achieve a pregnancy
• There is considerable debate today about whether PGS is in fact a technique that can help and increase pregnancy rates. This is because of the mosaicism that embryos show, which can lead to a misdiagnosis. For this reason, research is under way to establish whether preimplantation genetic screening can become an established part of assisted reproduction and genuinely help infertile couples.

As regards PGS technology, two newer methods that offer considerably greater diagnostic accuracy are comparative genomic hybridisation (CGH) and next-generation DNA sequencing (New generation sequencing, NGS).