Advantages and disadvantages of PGD
In case of the molecular cytogenetic examination it is necessary to take into account the possibility of an error as a consequence of the chromosomal mosaicism, when a proportion of the embryonic cells is normal while the other cells contain a pathological chromosomal feature. Up to 50 % of embryos may be disunited in their chromosomal equipment. For any further embryonic development the percentage representation of normal and pathological chromosomal equipment is important. It has been demonstrated that a small pathological clone is able to eliminate the embryo during the further development.
Thus the embryo could develop into a foetus with a fully normal chromosomal equipment, however, during the preimplantation genetic examination, it could be characterised as a pathologic one. The risk of this erroneous diagnosis is about 1.3 %. The opposite situation can also arise. The embryo could be denoted as a normal one, however, its chromosomal equipment would be pathologic. The risk of this error is about 4.3 %.
In this context we cannot miss a special group, which is formed by embryos with fully chaotic genetic equipment. Each of the cells of this so-called chaotic embryo carries a different chromosomal aberration. As a different type there are embryos with tetraploid or octaploid number of chromosomes.
The increased representation of embryos with abnormal chromosomal equipment – aneuploid equipment, mosaics, chaotic caryotype – is found in infertile couples.
In case of carriers of structural chromosomal aberrations it is possible, with the help of PGD, to mark the embryos with a chromosomal imbalance. However, it is not possible to safely distinguish the completely sound embryos from the ones being relatively healthy but carrying the genetic aberration, as one of the parents. The carriers of structural chromosomal aberrations (for example, translocations) are, in addition to being affected by the creation of genetically imbalanced gonadal cells due to the given chromosomal aberration, also burdened by the so-called interchromosomal influence of their chromosomal aberration to other chromosomes, which can lead to the origin of further chromosomal defects (especially aneuploidy of other chromosomes).
In case of PGD, therefore, it is necessary to count with the risk, that the whole IVF cycle may not be completed with an embryo transfer into the uterus. The examination may be foiled by a low number of cleavaging embryos, incorrect manipulation of the collected embryonic cell or a contradictory result of the PGD. As it was mentioned above, the genetic analysis may give incorrect results.
The PGD examination is always focused only on the selected type of chromosomal defect and it is not excluded, that in case of pregnancy a suspicion for another genetic defect will be expressed.
Although PGD cannot fully substitute the genetic examination of the foetus at a later stage of pregnancy (for example amniocentesis), it increases the chances of successful implantation of the embryo into the uterus, reduces the risk of spontanneous abortions at a later stage, and particularly significantly reduces the risk of genetic affection of the foetus.