Increased expansion and decreased contraction of embryos corresponds to increased clinical pregnancy rates in single FET cycles

Objective To investigate the possible association between expansion or contraction of blastocoeles after thawing with ongoing clinical pregnancy rates in frozen embryo transfer (FET) cycles. We hypothesize a greater change in blastocele size between thaw and transfer will correspond with an increase in pregnancy rate. Design Retrospective Study Materials and Methods A total of 257 single FET cycles from November 2017 to January 2019 were included. Patient ages ranged from 24 to 48 years. Programmed FET priming was performed with exogenous estrogen and progesterone. 67 embryos that underwent PGT-A (Preimplantation-Genetic Testing for aneuploidy) and 9 donor egg cycles were included. All blastocysts were cultured in trigas incubators ranging from 20 minutes to 4 hours 42 minutes. Pictures of each blastocyst were taken utilizing the Hamilton Thorne Zilos laser software (Beverly, MA) immediately after thaw and before transfer. The longest portion of the embryo was measured in µm using the ruler tool on the laser by one of two laboratory technicians. Pregnancy was defined by a positive HCG 9 days after embryo transfer and ongoing clinical pregnancy was defined by the presence of fetal cardiac activity at 6 ½ weeks. Wilcoxon rank sum tests were used to access differences in change parameters. Results Using the two measurements and their time difference we analyzed the amount of expansion and contraction and the rate of expansion and contraction. The 186 women that had embryos that expanded between thaw and transfer were more likely to have ongoing pregnancies than those whose embryos did not expand (median expansion 27.0µm vs. 19.6µm, p=0.036). The median percent change of expansion was also significantly different between groups (p=0.028) (Table 1). Among the 70 embryos that contracted, there was a trend between more distance contracted and lower ongoing pregnancy (p=0.075) and percent change of contraction and ongoing pregnancy (p=0.058), but neither reached significant. Conclusions We found a significant increase in ongoing pregnancy rates from embryos with greater expansion rates and percent changes in expansion. We also found a trend between increased pregnancy rates and smaller amounts of embryo contraction and percent change of contraction, however, neither reached significance. According to this study, a larger degree of embryo expansion post thaw has a better chance of ongoing pregnancy. Additional studies are needed to determine if smaller degrees of contraction is suggestive of ongoing pregnancy outcome. Disclosures None. Funding None.