Long-term storage of vitrified oocytes does not affect pregnancy and live birth rates: analysis of 5362 oocyte donation cycles

Research question: Does long-term storage of vitrified oocytes affect laboratory and reproductive outcomes after intracytoplasmic sperm injection?Design: Retrospective cohort study including 41,783 vitrified-warmed oocytes from 5362 oocyte donation cycles between 2013 and 2021. Five categories of storage time were established to analyse its effect on clinical and reproductive outcomes (<= 1 year [reference group], 1-2 years, 2-3 years, 3-4 years and >4 years).Results: The mean number of warmed oocytes was 8.0 +/- 2.5 oocytes. Oocyte storage time ranged from 3 days to 8.2 years (mean: 0.7 +/- 0.9). Mean oocyte survival (90.2% +/- 14.7% overall) did not significantly decrease with longer storage time after adjusting for confounders (88.9% for time >4 years, P = 0.963). A linear regression model did not show a significant effect of oocyte storage time on fertilization rate (about 70% in all time categories) (P > 0.05). Reproductive outcomes after the first embryo transfer were statistically comparable across storage times (P > 0.05 for all categories). Longer term oocyte storage (>4 years) did not affect the chances of clinical pregnancy (OR 0.700, 95% CI 0.423 to 1.158, P = 0.2214) or live birth (OR 0.716, 95% CI 0.425 to 1.208, P = 0.2670).Conclusions: Oocyte survival, fertilization rate, pregnancy and live birth rates are not affected by the time spent by vitrified oocytes in vapourphase nitrogen tanks.