THE THIRD PGD TYPE. A BIOPSY OF TROPHECTODERM

A biopsy of trophectoderm is performed at the blastocyst stage.

The ADVANTAGES of a biopsy on day 5-6 are:

• A minimal risk of embryo damage since trophectoderm cells compose extraembryonic tissues;
• The possibility of a biopsy of a larger number of cells without any detriment to further development; an increased accuracy of diagnosis;
• A low risk of embryo mosaicism (self-correction);
• The biopsy is carried out in all embryos obtained in the IVF cycle that have reached the blastocyst stage (the analysis of a lesser number of embryos – only of those having potential for development);
• A higher embryo implantation rate, the possibility of a better "preparation" of the endometrium for implantation in the next cycle;

The LIMITAIONS are:

• Mosaicism confined by the placenta (in developing embryos the difference in the chromosomal complement between trophectoderm cells and inner cell mass cells is 5-10% (Brezina et al., 2012, Johnson et al., 2010);
• The time for cytogenetic studies is limited in case of a transfer in the current cycle, but embryos can be vitrified and a transfer can be performed in the next cycle, and this shortcoming turns into an advantage;
• A relatively high risk of transfer cancellation;
• Technical complexity;
• In case of PGD by means of FISH – fixation difficulties.

The RISKS of pre-implantation diagnosis:

• The risk of accidental embryo damage (up to 5 %);
• Wrong diagnosis (up to 10%), cellular mosaicism in pre-implantation embryos can be observed in up to 50% of cases (Munne S., Sandalinas M., Escudero T. et al. 2002, Delhanty J. et al, 1997);
• A 3.5% probability of diagnosing an embryo with a pathology as healthy (FISH);
• A 10% probability of diagnosing a healthy embryo as an embryo with a pathology (FISH);
• The cancellation of an embryo transfer because according to PGD results a pathology is found in all embryos (up to 20%);

The accuracy of diagnosis depends on the resolution of the methods as well as on their limitations and the probability of an error. The choice of a method is governed by indications for PGD and the peculiarities of conducting an IVF cycle with PGD