Post-warming re-expansion and contraction dynamics of vitrified blastocysts: shrinkage magnitude and timing are correlated with pregnancy outcome.

OBJECTIVE

To analyze the relationship between blastocyst postwarming dynamics, specifically re-expansion and contractions, with implantation and live birth (LB) rates and overall embryo viability after vitrification and warming.

DESIGN

Retrospective cohort study.

SETTING

University-affiliated private in vitro fertilization center.

SUBJECTS: A total of 846 warmed blastocysts from patients undergoing frozen cycles (February 2022 to May 2024).

INTERVENTION: None.

MAIN OUTCOME MEASURES: Postwarming re-expansion dynamics and contraction types-pre-re-expansion contractions (PRCs) and mid-re-expansion contractions (MRCs) (pumping or collapsing)-inner cell mass size, and cytoplasmic strings (CSs) were analyzed for their correlation with implantation and LB rates. Additionally, logistic regression models incorporated these variables alongside conventional and artificial intelligence (AI)-based embryo quality assessments.

RESULTS

Of the 846 vitrified blastocysts, 97.5% survived warming, of which 95.5% were able to initiate re-expansion. Implantation and LB rates were higher (46.8% and 46.9%, respectively) in re-expanded vs. non-re-expanded blastocysts (18.2% for both outcomes). Faster re-expansion speed, thinner zona pellucida, larger final blastocyst size, and larger inner cell mass area were positively correlated with successful treatment outcomes. Moreover, CSs were identified in 47.4% of embryos and associated with implantation (50.5% vs. 43%). The presence of PRCs was associated with higher implantation rates (50% vs. 42.1%). In contrast, the presence of collapse (MRC shrinkage, >15%) reduced implantation (from 47.3% to 31.8%) and LB (from 41.5% to 27.3%) rates. Pumping (MRC shrinkage, <15%) embryos were more likely to hatch naturally after warming (18.8% vs. 8.7%). Postwarming contractions were correlated with embryo morphological quality assessed by embryologists and an image-based AI software.

CONCLUSION

Postwarming re-expansion and contraction dynamics are critical indicators of embryo viability and competence. Specifically, strong MRCs or collapses negatively affect implantation and LB potential, whereas minor contractions or pumping may assist in successful hatching. Faster re-expansion, PRC presence, and CS improve implantation outcome, suggesting a need to take these data into account when evaluating warmed embryos and to integrate contraction metrics into AI-assisted embryo selection. Thus, enhanced monitoring of these dynamics may guide embryo selection and clinical decision-making, ultimately optimizing pregnancy success rates.