EFFECT OF RE-EXPANSION IN FROZEN-THAWED EMBRYO TRANSFERS ON PREGNANCY OUTCOMES

Abstract

To evaluate the effect of blastocyst re-expansion on frozen-thawed embryos at time of transfer on pregnancy outcomes. Retrospective cohort study. Patients undergoing frozen-thawed embryo transfer (FET) from January 2017 to March 2020 in our center were included. Following a standard FET protocol, vitrified blastocysts were rewarmed on the day of ET and images of the thawed embryo were taken prior to transfer. Images were reviewed by at least two embryologists and the degree of expansion at least 1 hour post thaw at time of FETs was documented and included: 1) fully re-expanded 2) partially re-expanded 3) non re-expanded. Primary outcomes included: high quality embryo, defined by minimum grades of B for both trophectoderm and inner cell mass by Gardner’s blastocyst grading scale; pregnancy; clinical pregnancy; miscarriage; and, ongoing pregnancy and live birth. Statistical analyses were done using ANOVA for continuous data and Chi Square for dichotomous outcomes. A total of 645 single FET cycles were analyzed in the study. There were 301 fully re-expanded (46.7%), 274 partially re-expanded (42.4%), and 70 non re-expanded (10.9%) embryos at the time of transfer. The mean age, BMI, and endometrial thickness was not significantly different between the groups (p=NS). There was a significantly higher rate of embryo quality (56.8% and 58.0% vs. 44.3%; p=0.034), pregnancy rate (68.4% and 68.2% vs. 55.7%; p=0.048), clinical pregnancy rate (62.1% and 62.8% vs. 55.7%; p=0.030), and ongoing or live birth rate (50.2% and 51.8% vs. 38.6%; p=0.048) in frozen-thawed embryos that were fully or partially expanded at the time of transfer versus non re-expanded, respectively. No difference was noted in miscarriage rate between the groups (10.6%, 10.9%, 10.0%; p=0.9). The majority of thawed embryos were at least partially re-expanded (89.1%). The non re-expanded group has fewer high quality embryos (56.8% and 58.0% vs. 44.3%; p=0.034). When the high quality embryos were analyzed alone, there were no significant differences (p=NS) in pregnancy rates (70.2% vs. 72.5% vs. 64.5%), clinical pregnancy rates (64.9% vs. 67.5% vs. 54.8%), ongoing or live birth rates (52.6% vs. 58.8% vs. 41.9%) or miscarriage rate (10.0% vs. 8.8% vs. 12.9%) between the fully, partially, and non re-expanded groups, respectively. Transferring a non-re-expanded blastocyst, as compared to a fully or partially expanded blastocyst, at the time of FET is associated with a lower ongoing pregnancy and live birth rate in our study. However, embryo quality appears to have a greater effect on pregnancy outcomes than re-expansion, as there were no differences in outcomes between the groups when considering only high-quality blastocysts.