Blastocoele re-expansion at time of frozen embryo transfer affects clinical outcome following vitrification and warming

INTRODUCTION: While previous studies have correlated blastocyst expansion with pregnancy, there is limited evidence regarding post-warming characteristics and specifically the capability of the vitrified and warmed blastocyst to re-expand fully to the pre-vitrification state. OBJECTIVE: This study aimed to examine the effect of blastocyst re-expansion on clinical pregnancy rates (CPRs) in frozen embryo transfer (FET) cycles. MATERIALS AND METHODS: In this retrospective cohort, FET patients were stratified into "collapsed" and "re-expanded" groups and further categorized into good (≥ 3BB) or low (<3BB) embryo quality at time of vitrification. Patients were age-matched on a 1:2 ratio for comparison between the collapsed (n=165) and re-expanded (n=330) groups. The primary outcome of clinical pregnancy rate (CPR), in addition to secondary pregnancy outcomes, was reported. Chi-square tests of independence and Cramer's V test were performed to examine the association between CPR of collapsed versus re-expanded blastocysts; embryo quality; and age (< 38 years vs ≥ 38 years). Odds ratios with 95% confidence intervals were reported. RESULTS: A total of 5,996 patients underwent frozen embryo transfer (FET) between 2005 and 2012, of whom 165 (2.8%) received a collapsed blastocyst. Patients were age-matched in a 1:2 ratio in comparison to patients who implanted a re-expanded blastocyst (n = 330). Clinical pregnancy rate (CPR) was 28.5% in the re-expanded group compared with 15.8% in the collapsed group (OR = 2.145, 95% CI [1.324, 3.473]). There was a significant association between CPR and blastocyst expansion, χ²(1, N = 496) = 9.901, p = .002, with a small effect size (V = 0.14). CPR was also significantly associated with embryo quality (p = .017) and age (p < .001), regardless of embryo expansion. CONCLUSION: Our findings suggest that the transfer of a collapsed blastocyst during FET is associated with a lower CPR than a re-expanded blastocyst. Despite these differences, a viable pregnancy can be achieved after the transfer of a collapsed blastocyst, especially in younger patients with good quality embryos at the time of vitrification.