Embryo long-term storage does not affect ART outcome: analysis of 58001 vitrified blastocysts over an 11-year period.

Abstract

BackgroundRecently the potential detrimental effect that the duration of storage time may have on vitrified samples has raised some concern, especially when some studies found an association between cryo-storage length and decreased clinical results. ObjectiveThis study aimed to evaluate the effects of the storage time length of day-5 vitrified blastocysts in two study groups: freeze-all cycles and non elective frozen embryo transfers (FET). Study designThis was a retrospective study including 58001 vitrified/warmed day-5 blastocysts from two different populations according to the reason for FET. Elective FET comprised freeze-all cycles (N=16615 blastocysts and 16615 patients) where only single embryo transfers (SET) and only the first FET were included. The non elective FET group included 41386 embryos from 25571 patients where FET took place using supernumerary embryos after fresh embryo transfer. All the possible FETs were included. Both SET and double embryo transfers (DET) were included. Donor and autologous oocytes were used. The period covered by this study was 11 years. The blastocysts sample was clustered into deciles which provided specific storage duration categories. Main outcome was live birth rate (LBR) and secondary outcomes were embryo survival, miscarriage, clinical and ongoing pregnancy rates according to storage duration. The impact of storage time was assessed by univariable analyses in both groups. The comparison was made between each decile to the last one. A multivariable logistic regression analysis was conducted including the variables with significant association found in the univariate analysis. Student's t-test and Chi squared tests, or an analysis of variance were used wherever appropriate. P < 0.05 was considered statistically significant. ResultsThere were statistical differences in baseline characteristics of patients included in the study groups. Storage durations ranged from ≤ 0.67 to ≥ 4.34 and from ≤ 1.8 to ≥ 34.81 months in freeze-all and non elective FET respectively. Embryo survival did not show statistical differences across the categories of storage time in freeze-all and non elective FET groups. Statistical differences were found for the LBR across some, but not all the subgroups of storage duration. The multivariable analysis showed no association between storage time and the LBR in both groups (NS). Blastocyst quality, body mass index (BMI), number of retrieved oocytes, endometrial preparation, male factor and uterine factor were related to the drop in the LBR in the freeze-all group (p<0.05). In the non elective FET group the variables that showed significant association with the LBR were, age at retrieval and at FET, type of FET (SET or DET), number of retrieved oocytes, BMI, endometrial preparation, origin of sperm sample and female factor. ConclusiónThis large study demonstrated no association between storage time and clinical outcome. Other variables such as patient's age, embryo quality, BMI and etiology, are somewhat responsible for impacting outcome. This provides evidence for the safety of embryo vitrification, even after long storage periods. This is reassuring for both IVF practitioners and the patients undergoing FET of either elective or non elective embryos.