P-198 Time to maximum blastocyst expansion (tmBE) may represent a novel metric for assessing embryo competence
Authors: Aranda I , Fraire-Zamora J  J , Zamora M  J , Torra-Massana M , Rodriguez A , Popovic M 
Published: July 2024

Abstract

Study question
Can the time to achieve maximum blastocyst expansion (tmBE) and the frequency of spontaneous collapses serve as useful indicators of embryo competence?

Summary answer
Compared to the frequency of spontaneous collapses, tmBE was a better indicator of embryo competence, potentially serving as a valuable metric for embryo selection.

What is known already
Identifying the most viable blastocyst for transfer is essential for success in assisted reproduction. In scenarios with good prognosis, such as oocyte donation, blastocyst selection may require a more nuanced approach, given the high number of available good quality embryos. Exploring indicators of embryo competence beyond traditional morphology-based criteria may be beneficial, however an in-depth understanding is still lacking. Blastocyst expansion and contraction dynamics have been suggested to play a role in developmental and chromosomal fitness. However, their predictive value has not been investigated across a wide range of patient populations.

Study design, size, duration
This is a single-center retrospective study of 287 blastocysts transferred in autologous (n = 88) and heterologous (n = 199) cycles, performed between September 2019 and September 2022. Only fresh single embryo transfers were considered. We compared blastocysts across two groups: advanced maternal age (AMA) patients (&gt;35 years old, n = 88) and oocyte donors (&lt;35 years old, n = 199). We evaluated well-established indicators of embryo quality, as well as novel metrics, including tmBE and the frequency of spontaneous collapses.

Participants/materials, setting, methods
Conventional indicators of embryo quality included blastocyst grade and morphokinetic timings (tPNf, t2, t3, t4, t5, t8, tSB and tB). tMBE was defined as the maximum time (hpi) required for the blastocyst to reach its largest diameter, extending beyond tB. Blastocyst contractions were measured as a decrease and subsequent recovery of the blastocoel area over time. ImageJ was employed for size measurements. p&lt;0.05 was considered statistically significant after Two-tailed Student’s t-test or Chi-square test.

Main results and the role of chance
We observed a significant difference in live birth rates between oocyte donors and AMA patients (40% vs 18%, p=0.0004). Nevertheless, there were no differences across blastocyst expansion scores between oocyte donors and AMA patients (5= 58% vs 65%; 4= 38% vs 30% and 3= 4% vs 5%, p &gt; 0.05), nor in their morphology score (Excellent= 57% vs 61%; Good= 24% vs 18%; Average= 15% vs 16% and Poor= 4% vs 5%). In terms of morphokinetics, when comparing oocyte donor and AMA embryos, the only significant difference was observed at tPNF (22.0±2.4 vs 22.6±2.3; p= 0.04). No differences were found at t2, t3, t4, t5, t8, tSB and tB. Additionally, the frequency of spontaneous collapses showed no significant differences between the oocyte donor and AMA group (1.4±1.2 vs 1.5±1.0, p=0.66). However, blastocysts from oocyte donors reached tmBE significantly faster than AMA embryos (115.0±4.8 vs 117.2±3.4; p = 0.0003). When these variables were further correlated to clinical outcomes, the frequency of spontaneous collapses did not appear to influence live birth in either group. Notably, however, tmBE was significantly shorter for embryos resulting in a live birth, across both the oocyte donor (113.5 vs 115.4, p = 0.04) and AMA group (115.6 vs 117.6, p = 0.04).

Limitations, reasons for caution
This is a single center retrospective study with a relatively small sample size, especially in the AMA group. Caution should be made when generalizing results to other patient populations, as confounding were not taken into consideration. No data on the chromosomal status of embryos was available.

Wider implications of the findings
Our results suggest that tMBE may serve as a reliable indicator of embryo potential, allowing a more nuanced embryo selection, particularly in cases with a high number of good quality embryos. This study opens avenues for further research into novel metrics for assessing embryo competence

Trial registration number
not applicable