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Suboptimal oocyte cohort maturity impairs the implantation potential of euploid blastocysts

petri dish

Authors: Anderson RE, JB Whitney, CM Rios, K Balloch, MC Schiewe

Study question: Can oocyte cohort maturity predict the implantation potential of euploid blastocysts?  Do stimulation protocols influence aneuploidy and the ability of euploid blastocysts to implant?

Summary answer: Ovarian hyperstimulation protocols yielding reduced oocyte maturity rates(<70%) are associated with lower (P<0.05) blastocyst development, euploidy and implantation rates, likely correlated to incomplete cytoplasmic maturation.

What is known already: Controlled ovarian hyperstimulation in IVF has greatly improved the success of assisted reproduction.  Stimulation protocols, medications, duration and LH trigger timing are used by physicians to optimize mature oocyte production, and have been studied rigorously to identify their effect on oocyte quality and pregnancy.  Oocyte quality is difficult to independently quantify, while nuclear maturation of oocyte cohorts indexes overall stimulation effectiveness. Applying PGT-A technology, euploid blastocysts are transferred with the expectation of a constant implantation potential across age groups. However, it is unknown what role oocyte cytoplasmic preparedness (i.e., maturation) could play on the implantation potential of euploid embryos.

Study design, size, duration: A 5-year (2014-2018) retrospective analysis of 1358 autologous and donor stimulation cycles using PGT-A and 1002 vitrified-thawed euploid embryo transfers was examined.  Nuclear maturity was defined by the presence of a polar body at ICSI and cycle cohort comparisons were separated by Group 1: ≥70%(n=1124) and Group 2: below 70% (n=234). Data comparisons using t-test and chi-square were performed for cycles failing to produce blastocysts, cycles resulting in normal embryos, aneuploidy rates and implantation success.

Participants/materials, setting, methods: Single physician clinic stimulated 1138 patients for 1358 cycles having a cohort of oocytes retrieved.  All ICSI-derived zygotes were grown to the blastocysts stage for PGT-A and vitrification.  Only the first transfer attempt was counted in our analysis. Stimulation protocols varied but were predominantly antagonist based.  Oocytes were retrieved 35-36 hours post-hCG trigger and cumulus complexes denuded 2-3 hours post-retrieval.  Patient’s percent mature oocytes were calculated based on Metaphase II status upon denuding.

Main results and the role of chance: Average patient age was 37 and 38 for groups 1 and 2, respectively. Oocyte cohort maturity of 70% or greater (Group 1) occurred in 83% of the cycles, with 17% considered suboptimal at less than 70% maturity (Group 2).  Embryo development comparisons focusing on blastocyst yield, aneuploidy and whether a normal embryo was produced/cycle exhibited a lower (p<0.01) blastocyst yield (mean=2.7) in Group 2 compared to Group 1 (mean=5.7 blastocysts).  Furthermore, Group 2 had a statistically higher aneuploidy rate of 61% and more cycles (41%) failing to result a normal embryo, in contrast to Group 1 having an aneuploidy rate of 54% and 23% of its cycles not resulting in a normal embryo.  Most importantly, the implant ability of euploid embryos was lower (p<0.05) for Group 2 at 61% compared to 70% in Group 1.  The reduced cytoplasmic preparedness of Group 2 oocytes led to developmental incompetence as indicated by 18% more cycles failing to develop a euploid blastocyst and significantly more aneuploidy.  The issue of reduced cytoplasmic maturation of Group 2 oocytes was further manifested when they produced top quality euploid blastocysts with a lower(p<0.05) implantation potential upon transfer.

Limitations, reasons for caution: Multiple factors influence ovarian stimulation responsiveness and oocyte cohort maturity, nonetheless 5 years of data has identified a group of cycles that increased the risk of aneuploidy and produce euploid embryos with reduced implantation potential.  These significant trends suggest possible adverse developmental events associated to related incomplete cytoplasmic maturation.

Wider implications of the findings:  Many factors can influence controlled ovarian stimulation, oocyte maturity and subsequent negative cycle outcomes.  Further investigations and evidence of oocyte-cellular cytoplasmic proteomics, metabolomics and gene activation/regulation is needed to better identify how sub-optimal maturity can adversely influence subsequent embryo viability and healthy, term live birth success.