P-712 Optimal interval time between embryo freezing and embryo transfer in women undergoing freezing-all embryo transfer
Authors: Hu K.L , Sarah H , Li R , Ben W M 
Published: June 2022

Abstract

Study question
In women having a freeze all cycle, does the time frame between freezing and the final embryo transfer matter?

Summary answer
Women undergoing a freeze all embryo transfer cycle may benefit from a three to four months interval between embryo freezing and embryo transfer.

What is known already
A freeze all embryo cycle strategy is increasingly employed in assisted reproductive technology (ART) practice. The time needed for the endometrium to the optimal receptivity after ovarian stimulation and the effects of prolonged vitrification on embryo viability and subsequent implantation potential remains unclear.

Study design, size, duration
A single-center retrospective cohort study was performed in Peking University Third Hospital. Women who underwent their first vitrified-warmed cycles from January 2013 to December 2019 were included.

Participants/materials, setting, methods
Women were divided into eight categories according to the frozen time of transferred embryos: Group 1, 0.8-1.0 months; Group 2, 1.1-2.0 months; Group 3, 2.1-3.0 months; Group 4, 3.1-4.0 months; Group 5, 4.1-5.0 months; Group 6, 5.1-6.0 months; Group 7, 6.1-12.0 months; Group 8, 12.1-max months. Multivariate logistic regression was used to test the risk of factors with the expression of crude odds ratios (OR) and adjusted OR (aOR) with 95% confidence intervals (CI).

Main results and the role of chance
A total of 14,928 women who underwent freeze-all treatment were eligible for analysis. Stratifications of embryo storage time showed that group 3, group 4, and group 5 were associated with the higher live birth rate (42%, 41%, 42%) with live birth rates in group 1 at 35% (aOR=0.82 (0.68, 1.02)), group 2 at 38% (aOR=0.91 (0.81, 1.01)), group 6 at 39% (aOR=0.94 (0.79, 1.12)), group 7 at 34% (aOR= 0.83 (0.71, 0.96)), group 8 at 30% (aOR= 0.68 (0.52, 0.90)). The results were confirmed by an inverted U curve in the restricted cubic splines before as well as after adjustment for covariables, which suggested that an embryo storage time of 3-4 months was associated with the highest live birth rate. Subgroup analysis demonstrated that the inverted U curve relationship between embryo storage time and live birth rate was enhanced in women with the high response, with a significantly higher live birth rate in group 5 in women with high response (aOR= 1.19 (1.00, 1.41)). Once pregnancy had occurred, there was no difference in neonatal outcomes between the groups, except a higher risk of preterm birth in group 1 (13%).

Limitations, reasons for caution
The main limitation of our study was its retrospective nature. Even we have adjusted available covariates in the multivariate analysis and have conducted several sensitivity analysis, there is the possibility of residual confounding and unmeasured confounders. Our findings should be tested in large randomized clinical trials.

Wider implications of the findings
Women may benefit from a 3-4 months delay prior to embryo transfer in freeze-all cycles, particularly in women with number of oocytes retrieved higher than 20. Prolonged storage time of greater than 6 months was associated with lower pregnancy rates but not adverse perinatal outcomes.

Trial registration number
NA