P-728 Impact of calling mosaicism for a less stringent threshold on clinical pregnancies in more than 6000 PGT-A cycles
Authors: Cetinkaya M , Tufekci M A , Cinar Yapan C , Yelke H , Colakoglu Y , Kahraman S 
Published: June 2023

Abstract

Study question
How does calling mosaicism for a higher threshold affect euploidy, the chance of finding at least one euploid embryo and clinical pregnancy?

Summary answer
Increasing the threshold significantly increased euploidy percentage and the probability of finding at least one euploid embryo, as expected, but also the pregnancy outcomes.

What is known already
NGS technology has shed light on the occurrence of chromosomal mosaicism at the preimplantation stage of development. Evidence regarding clinical outcomes after transfer of embryos with putative mosaic results are accumulating (ESHRE Working Group on Chromosomal Mosaicism, 2022).
A series of studies reporting the birth of healthy babies after the transfer of embryos with a chromosomal mosaic result on PGT-A (Greco et al., 2015; Kahraman et al., 2020; Viotti et al., 2019 and 2021), have been published. Data suggested lower implantation rates and higher miscarriage rates when mosaics were compared with euploid embryo transfers.

Study design, size, duration
This retrospective study covers a period from January 2017 to October 2022 and includes 6063 PGT-A cycles. A total of 18425 blastocysts were biopsied and analyzed with NGS. From January 2017 to December 2020, the cut-off value for reporting mosaicism and euploidy was %20; the rate of which was increased to 30% after January 2021. Chromosomes with PGT-A profiles that deviate from 2 copies by less than 30% were reported as normal.

Participants/materials, setting, methods
A total number of 12751 and 5674 blastocysts were biopsied between January 2017-December 2020 and January 2021-October 2022, respectively. PGT-A was performed on Ion Torrent S5 (Thermo Fisher Scientific).
The mean female age for 20% and 30% cut-off groups were 36.7+-5.1, 35.9+-5.2, respectively. As this age difference was statistically significant, further analysis was performed in age subgroups &lt; =38 and &gt;38.

Main results and the role of chance
For PGT-A cycles with female age &lt; =38, cycles with at least one euploid embryo in the 30% cut-off group increased from 74.0% to 77.5% whereas cycles with at least one mosaic embryo decreased from 40.8% to 37.0% (p = 0.0212; p = 0.0274). Cycles with at least one mosaic embryo without any euploid embryos decreased by 2% (9.2% to 7.2%) (p = 0.0522). The euploidy percentage out of all tested biopsies was significantly increased from 40.4% to 45.8% (p = 0.0001).
For cycles with female age &gt;38, ratio of PGT-A cycles with at least one euploid embryo increased from 28.1% to 30.3% in the 30% cut-off group but this was not statistically significant (p = 0.28). Cycles with at least one mosaic embryo were decreased from 15.1% to 10.8% and cycles with at least one mosaic embryo without any euploid embryo decreased from 9.5% to 5.6% in the 30% cut-off group (p = 0.006; p = 0.0018). The euploidy percentage was increased by 2.8% from 15.1% to 18.1% (p = 0.0115).
The pregnancy outcomes were significantly higher for euploid embryos with the 30% cut-off compared to 20%, as clinical pregnancy was 64.5% vs. 59.1% and clinical pregnancy loss was 7.9% vs. 12.4% (p = 0.0022; p = 0.0011). Similarly, ongoing pregnancy was 5.2% higher, but non-significant.

Limitations, reasons for caution
These are the results of a single ART center which is using the same protocols for ovarian stimulation and endometrial preparation in the two periods above mentioned. Although not expected, minor changes may have occurred; similar practices have to be reported.

Wider implications of the findings
Mosaicism is undeniably a true biological phenomenon. However, trophectoderm biopsy and NGS are prone to create an artefactual false mosaicism.
Relaxing the mosaicism calling rate by 10% may have added some better-quality blastocysts to the euploid pool giving the opportunity to embryologists of transferring blastocysts with higher implantation potential.

Trial registration number
Not applicable