Impact of thawing-loading interval of frozen embryo on pregnancy outcome: retrospective analysis

Abstract

Embryo transfer is a critical final step in the in vitro fertilization (IVF) cycle. Much attention has been paid to standardizing the procedures of embryo transfer to improve pregnancy outcome. Objective. To identify the impact of embryo thawing-loading interval (the time interval passing from thawing to loading of frozen embryo into the catheter) on pregnancy outcome. Results. The study was divided into quartiles according to thawing-loading interval in minutes (G I: <60; G II: 60–119; G III: 120–239; G IV: 240 and above). Numerical variables are presented as median and interquartile range, and inter-group differences are compared using the Mann-Whitney U test. Categorical variables are presented as numbers and percentages, and differences are compared using the chi-square test for trend. After adjustment for the age of women, time of previous failed ICSI (Intracytoplasmic sperm injection) trials, number and maturity of transferred embryos, a thawing-loading interval between 60–119 minutes (adjusted odds ratio = 2.222, 95% CI = 1.370 to 3.603, p-value = 0.001) or between 120–239 minutes (adjusted odds ratio = 1.924, 95% CI = 1.172 to 3.161, p-value = 0.010) was independently associated with increased probability of clinical pregnancy compared with a thawing-loading interval of <60 minutes. On the other hand, a thawing-loading interval of ≥240 minutes significantly decreased the probability of clinical pregnancy (adjusted odds ratio = 0.378, 95% CI 0.215 to 0.664, p-value = 0.001). Conclusion. After adjusting for potential confounders, this analysis found that a thawing-loading interval between 60–239 minutes was independently associated with better pregnancy outcomes compared with a thawing-loading interval of <60 minutes. On the other hand, a thawing-loading interval of ≥240 minutes was significantly associated with a worse outcome. Key words: embryo transfer, IVF (in vitro fertilization), frozen embryo, thawing-loading interval