O-193 Opening the black box: why do euploid blastocysts fail to implant? A systematic review and meta-analysis
Authors: Cimadomo D , Rienzi L , Conforti A , Forman E , Canosa S , Innocenti F , Poli M , Hynes J , Gemmell L , Vaiarelli A , Alviggi C , Ubaldi F M , Capalbo A 
Published: June 2023

Abstract

Study question
What are the embryonic, maternal, paternal, clinical and laboratory features associated with the live-birth-rate (LBR) after euploid blastocyst transfer?

Summary answer
Delayed blastulation, poor blastocyst quality, maternal age ≥38 years, obesity, previous repeated-implantation-failure (RIF), poor or multiple manipulations may reduce the LBR per euploid blastocyst transfer.

What is known already
Euploidy assessed at the blastocyst stage through comprehensive-chromosome-testing (CCT) technologies during IVF cycles with preimplantation genetic testing for aneuploidies (PGT-A) represents the strongest predictor of embryo implantation potential. Nevertheless, its positive predictive value hardly exceeds 50-60%. The current gap of knowledge on the causes of euploid blastocysts’ reproductive failure is known as “the black box of implantation”.

Study design, size, duration
Systematic search conducted up to August-2021 with the keywords “(blastocyst OR day 5 embryo OR day 6 embryo OR day 7 embryo) AND (euploid OR chromosomally normal OR preimplantation genetic testing) AND (implantation OR implantation failure OR miscarriage OR abortion OR live birth OR biochemical pregnancy OR recurrent implantation failure)”. Among 1608 items, prospective/retrospective studies and randomized controlled trials (RCTs) investigating associations with LBR and/or miscarriage rates (MR) after non-mosaic euploid blastocyst transfers were scrutinized.

Participants/materials, setting, methods
335 retrospective, 30 prospective papers and 7 RCTs were included. PRISMA guideline, PICO model, and ROBINS-I and ROB 2.0 scoring for bias, were adopted. Funnel plots and trim and fill methods were used to visually inspect bias regarding the LBR. Categorical data were combined with a pooled-OR. The random-effect model was used for meta-analyses. Between-study heterogeneity was addressed using I2. Whenever not suitable for the meta-analysis, the included studies were simply described for their results.

Main results and the role of chance
Inner-cell-mass C-grade (7 studies, OR:0.37, 95%CI:0.27-0.52, p &lt; 0.01), trophectoderm C-grade (9 studies, OR:0.53, 95%CI:0.43-0.67, p &lt; 0.01), overall blastocyst quality &lt;BB (8 studies, OR:0.4, 95%CI:0.24-0.67, p &lt; 0.01) developmental delay (18 studies, OR:0.56, 95%CI 0.49-0.63, p &lt; 0.01) and, by qualitative analysis, some morphodynamic abnormalities pinpointed through time-lapse microscopy (abnormal cleavages, spontaneous collapse, longer tM, tB, and blastulation) were all associated with poorer reproductive outcomes. Slightly lower LBR was reported for women ≥38 years (7 studies, OR:0.87, 95%CI:0.85-1.00, p = 0.05), while maternal BMI&gt;30 involved both lower LBR (2 studies, OR:0.66, 95%CI:0.55-0.79, p &lt; 0.01) and higher MR (2 studies, OR:1.8, 95%CI:1.08-2.99, p = 0.02). The experience of previous RIF was also associated with lower LBR (3 studies, OR:0.72, 95%CI:0.55-0.93, p = 0.01). By qualitative analysis, abnormal progesterone level prior to transfer impaired LBR and MR. Among clinical policies, vitrified-warmed transfer was more effective than fresh (2 studies, OR:1.56, 95%CI:1.05-2.33, p = 0.03). Lastly, multiple vitrification-warming cycles (2 studies, OR:0.41, 95%CI 0.22-0.77, p &lt; 0.01) and, by qualitative analysis, high biopsy cellularity may slightly impact the LBR, while simultaneous zona opening and biopsy protocol showed higher LBR than the day3 hatching-based protocol (3 studies, OR:1.41, 95%CI:1.18-1.69).

Limitations, reasons for caution
The associations outlined mostly issue from retrospective and/or few studies or studies with a limited sample size, therefore the level of evidence is low or very low. Moreover, some findings represent “prognosis without promise” because the poorer outcome of euploid blastocysts is not clinically actionable (e.g., women ≥38 years).

Wider implications of the findings
Future research should target: (i) the mechanisms involved in reproductive aging beyond de novo chromosomal abnormalities, and how environment and nutrition may accelerate or exacerbate their consequences; (ii) uterine evaluation and blastocyst-endometrial dialogue; (iii) standardization of embryo assessment and IVF protocols; (iv) additional (non-)invasive tools for embryo selection.

Trial registration number
http://www.crd.york.ac.uk/PROSPERO/ (registration number CRD42021275329)