PGS theory is based on 3 important facts:
- Human embryos are often aneuploid:
- at the age 30-35 30-40% of blastocysts are aneuploid
- at the age of 40-42 70-80% blastocysts are aneuploid [ Fragouli et al., 2012]
- Aneuploidy is almost always lethal – leads to developmental arrest of the embryo due to shortage or excess of genetic material [Scott at al., 2012]
- Influence of chromosome abnormalities on embryonic morphology is very insignificant. In other words a “good looking embryo” can be aneuploid. [Alfarawati et al., 2011]
Aneuploid embryos can be chosen for embryo transfer, because they looked as good or even better than their euploid siblings which sadly were not chosen for transfer. Very often transfer of aneuploid embryo in the uterus will result in no pregnancy at all or rather often in early pregnancy loss before 12 weeks. Only selected types of chromosome mistakes are compatible with development throughout the whole pregnancy and may result, for example, in live birth of a Down syndrome baby (trisomy 21) or Turner syndrome (45, XO).
Conclusion: A wrong chromosome number in an embryo is the major reason for a morphologically good embryo not being able to attach and/or progress to a live born baby.
Testing of all 23 chromosome pairs in “good looking“ blastocysts to choose those for transfer which do not only have the highest chance to attach, but also to stay and result in live birth.
This embryo testing is called Pre-implantation genetic screening = PGS.
Major principle of PGS for 23 chromosome pairs is to sort out such good in morphology embryos with a wrong chromosome number and to use them neither for fresh transfer nor for freezing.
PGS does not make your embryos better. It helps to choose the most viable embryos from those you have. PGS does not increase the cumulative result of one IVF stimulation including all the transfers fresh and frozen from one or more IVF cycles. But it increases the implantation rates, clinical pregnancy rates and live birth rates per embryo transfer. Hence PGS for 23 chromosome pairs reduces time you spend to achieve pregnancy and live birth.
The higher the age of the egg used to create pregnancy, the higher risk of it being abnormal, the higher risk to pick up an abnormal embryo for transfer based only on embryo morphology.
The higher the age of the women the higher her benefit from choosing an embryo for transfer based on both morphological evaluation and PGD for 23 chromosome pairs.
Conclusion: PGS for 23 chromosome pairs eliminates the negative effect of maternal age on implantation (if there are chromosomally normal embryos found and embryo transfer is possible)