by Chrysa Karakosta, Embryologist
Posted on 05 March 2018, 9:43 am
Preimplantation Genetic Screening (PGS), if required, does not necessarily automatically "ring the bell" of abnormality. To fully comprehend this testing, you need to know that PGS is a procedure performed on embryos created in the lab. Either on day 3 or day 5 of their development, PGS is used to determine the chromosomal status of an embryo by screening all 23 pairs of existing chromosomes.
Each individual has 46 chromosomes (23 pairs). The chromosomal pair is numbered 1-22 and the final two are the sex chromosomes X & Y. A normal female has two X chromosomes and a male an X and a Y chromosome. Therefore, each embryo inherits one copy of chromosomes from the egg and one pair from the sperm.
We cannot test the male and female partner and have a definite answer regarding the chromosomal abnormalities of a potential embryo, since many aneuploidies (abnormalities) occur during the fertilization process and hence why it is essential to test the embryo itself.
Conventional IVF/ICSI offers limited success, as an aneuploidy embryo that may have a normal morphological development in the embryological lab may not implant or even if it does implant may lead to an early miscarriage. Aneuploidy rates increase with the maternal age. Studies have displayed that in women less than 35 years old have aneuploidy rates of about 35% but in ages over 42 these can rise to 85%.
PGS is performed in cases of repeated unexplained implantation failure and advanced reproductive age. The goal is to reduce recurrent miscarriage rates along with the amount of time and costs inferred with multiple IVF cycles, select embryos that are chromosomally normal and increase the likelihood of a pregnancy with elective single embryo transfers.
Recent technologies, such as Next Generation Sequencing, allow us to sequence DNA and RNA in more quick, effective and less expensive way compared to the formerly used techniques, and as such have revolutionized the study of genomics and molecular biology.
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