STUDY QUESTION: To evaluate the impact of storage time after vitrification on embryo viability, pregnancy outcomes and neonatal outcomes. SUMMARY ANSWER: The prolonged storage time of vitrified embryos negatively affected pregnancy outcomes, including biochemical pregnancy rate, clinical pregnancy and live birth rate; but did not influence neonatal outcomes. WHAT IS KNOWN ALREADY: Although vitrification has been the fundamental tool of ART treatments in recent years, few studies have explored the influence of storage period after vitrification on embryonic and clinical outcomes. STUDY DESIGN, SIZE, DURATION: A retrospective study was performed among 24 698 patients with the first vitrified embryo transfer following a freeze-all strategy during the period from January 2011 to December 2017. PARTICIPANTS/MATERIAL, SETTING, METHODS: A total of 24 698 patients met the inclusion criteria and were grouped according to the storage time (11 330 patients in Group 1 with storage time <3 months, 9614 patients in Group 2 with storage time between 3 and 6 months, 3188 patients in Group 3 with storage time between 6 and 12 months and 566 in Group 4 with storage time between 12 and 24 months). The pregnancy outcomes and neonatal outcomes were compared between different storage time groups. Multivariate logistic regression and linear regression were performed to evaluate the independent effect of storage time on clinical outcomes, adjusting for important confounders. MAIN RESULTS AND THE ROLE OF CHANCE: After adjustment for potential confounding factors, the chance of biochemical pregnancy (Group 1 as reference; Group 2: adjusted odds ratio (aOR) = 0.92, 95% CI 0.87-0.97; Group 3: aOR = 0.83, 95% CI 0.76-0.90; Group 4: aOR = 0.68, 95% CI 0.56-0.81), clinical pregnancy (Group 2: aOR = 0.91, 95% CI 0.86-0.96; Group 3: aOR = 0.80, 95% CI 0.73-0.87; Group 4: aOR = 0.65, 95% CI 0.54-0.79) and live birth (Group 2: aOR = 0.89, 95% CI 0.85-0.95; Group 3: aOR = 0.83, 95% CI 0.76-0.91; Group 4: aOR = 0.59, 95% CI 0.48-0.72) significantly decreased with the increasing storage time, whereas the relationship between miscarriage, ectopic pregnancy and storage time did not reach statistical significance. In addition, there was no evidence of differences in adverse neonatal outcomes (preterm birth, low birthweight, high birthweight, macrosomia or birth defects) between groups. LIMITATION, REASONS FOR CAUTION: Our study was limited by the retrospective design from a single center, the conclusion from our study needs to be verified in further studies. WIDER IMPLICATIONS OF THE FINDINGS: This study provides new findings about the relationship between prolonged storage time of vitrified embryos and clinical outcomes and offers evidence for the safety of using long-stored embryos after vitrification. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by the National Natural Science Foundation of China (grant nos. 81903324, 81771533, 81571397, 81701523), National Key Research and Development Program of China (grant no. SQ2018YFC100163). None of the authors have any conflicts of interest to declare.
STUDY QUESTION: To evaluate the impact of storage time after vitrification on embryo viability, pregnancy outcomes and neonatal outcomes. SUMMARY ANSWER: The prolonged storage time of vitrified embryos negatively affected pregnancy outcomes, including biochemical pregnancy rate, clinical pregnancy and live birth rate; but did not influence neonatal outcomes. WHAT IS KNOWN ALREADY: Although vitrification has been the fundamental tool of ART treatments in recent years, few studies have explored the influence of storage period after vitrification on embryonic and clinical outcomes. STUDY DESIGN, SIZE, DURATION: A retrospective study was performed among 24 698 patients with the first vitrified embryo transfer following a freeze-all strategy during the period from January 2011 to December 2017. PARTICIPANTS/MATERIAL, SETTING, METHODS: A total of 24 698 patients met the inclusion criteria and were grouped according to the storage time (11 330 patients in Group 1 with storage time <3 months, 9614 patients in Group 2 with storage time between 3 and 6 months, 3188 patients in Group 3 with storage time between 6 and 12 months and 566 in Group 4 with storage time between 12 and 24 months). The pregnancy outcomes and neonatal outcomes were compared between different storage time groups. Multivariate logistic regression and linear regression were performed to evaluate the independent effect of storage time on clinical outcomes, adjusting for important confounders. MAIN RESULTS AND THE ROLE OF CHANCE: After adjustment for potential confounding factors, the chance of biochemical pregnancy (Group 1 as reference; Group 2: adjusted odds ratio (aOR) = 0.92, 95% CI 0.87-0.97; Group 3: aOR = 0.83, 95% CI 0.76-0.90; Group 4: aOR = 0.68, 95% CI 0.56-0.81), clinical pregnancy (Group 2: aOR = 0.91, 95% CI 0.86-0.96; Group 3: aOR = 0.80, 95% CI 0.73-0.87; Group 4: aOR = 0.65, 95% CI 0.54-0.79) and live birth (Group 2: aOR = 0.89, 95% CI 0.85-0.95; Group 3: aOR = 0.83, 95% CI 0.76-0.91; Group 4: aOR = 0.59, 95% CI 0.48-0.72) significantly decreased with the increasing storage time, whereas the relationship between miscarriage, ectopic pregnancy and storage time did not reach statistical significance. In addition, there was no evidence of differences in adverse neonatal outcomes (preterm birth, low birthweight, high birthweight, macrosomia or birth defects) between groups. LIMITATION, REASONS FOR CAUTION: Our study was limited by the retrospective design from a single center, the conclusion from our study needs to be verified in further studies. WIDER IMPLICATIONS OF THE FINDINGS: This study provides new findings about the relationship between prolonged storage time of vitrified embryos and clinical outcomes and offers evidence for the safety of using long-stored embryos after vitrification. STUDY FUNDING/COMPETING INTEREST(S): This work was supported by the National Natural Science Foundation of China (grant nos. 81903324, 81771533, 81571397, 81701523), National Key Research and Development Program of China (grant no. SQ2018YFC100163). None of the authors have any conflicts of interest to declare.