Preimplantation genetic screening (PGS)
Aneuploidy in embryos (wrong number of chromosomes) may occur even from healthy partners. Euploids (embryos) receive a chromosome set of 23 pairs from each parent, i.e. by two copies of chromosomes. If set contains one extra chromosome (trisomy) or one missing chromosome (monosomy) or there any defects in DNA chain, embryo will be aneuploidy. This means that a child will have a genetic disease.
In natural conception genomic set is a “lottery”, it is not possible to define what genome a child will receive from the father and mother. In standard IVF procedure embryos are selected for the transfer visually, but damaged cells and “visually normal cells” have no difference. Preimplantation genetic screening allows to select healthy embryos without chromosome mutations.
There are different technologies of embryo screening (PGS). Previously FISH method was used to screen only 10 chromosome pairs. Now days two main PGS methods are used, these are:
NGS – Next-Generation Sequencing, this is more precise and cheap diagnosis.
CGH – Comparative Genomic Hybridization.
Both methods allow to examine all 23 pairs.
IVF with PGS testing is indicated in the following cases:
- recurrent miscarriage;
- unexplained infertility;
- missed abortion;
- previously failed ART procedures;
- husband has a poor sperm quality;
- women over 35;
- family members have hereditary diseases.
Before starting an embryo screening (PGS) it is necessary to do one of three types of the biopsy indicated below:
- Bipolar biopsy – polar bodies (tiny cells) are removed. Disadvantage of this method — obtained results contain information only on deviations occurred in the egg before fertilization and 30% of abnormalities cannot be detected.
- Blastomere biopsy – is done in on the 3rd day of embryonic development before embryo transfer. Disadvantage – decreases chances of implantation, there is a possibility of mosaicism presence (genetically different cells).
- Trophoblastic – cells are taken from the outer surface of the embryo. Advantage: no negative impact on embryonic development and other side effects. Disadvantages: duration of the testing (up to 24 hours), so embryo cryopreservation can be required before its transfer to the uterus. However, vitrification (ultra-fast freezing) allows to keep embryo quality and viability.
IVF with PGS process is carried out like a traditional IVF, it just includes an additional screening made before embryo transfer. In the meantime the procedure efficiency increases up to 30%, risk of spontaneous abortion at early gestational age decreases, treatment expenses decrease due to the reduced number of failed attempts.
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