PURPOSE: The aim of this study was to accurately describe outcome differences (cryo-survival, pregnancy rate and live birth rate, both per ET and cumulatively), between the vitrification method and slow-freezing method of surplus 2PN oocytes in an IVF program.
METHODS: In 2004, the freezing method for 2PN oocytes was changed from slow-cooling to vitrification. The data of 711 patients (timespan: 1/1999-7/2011; 410 vitrification and 301 slow-cooling events) undergoing a first IVF/ICSI cycles with freezing of 2PN oocytes were retrospectively analyzed. The outcome of one, the first, IVF cycle per patient was explored. The data were analyzed per freezing-thawing attempt as well as cumulatively per one complete IVF cycle, taking pregnancy occurrence after a fresh embryo transfer preceding the cryo-cycle(s) and other confounders (such as female age, elective vs. surplus 2PN cryopreservation) into account by means of exploratory regression analyses.
RESULTS: In the vitrification and slow-cooling group, 756 and 376, respectively, attempts of thawing 2PN oocytes were recorded. Each attempt of thawing 2PN oocytes showed statistically significantly higher mean cryo-survival rates after vitrification (effect size approximately 30-40%, with vitrification cryo-survival consistently above 90% in all thawing attempts). Furthermore, the incidence of "zero survival" was lower after vitrification (0.5 vs. 7.3%, p < 0.01). It is estimated that the odds of achieving a live birth per one IVF cycle (fresh and frozen transfers combined) with vitrification of 2PN oocytes is increased approximately 1.4-fold (OR of 1.405, 95% CI 0.968-2.038; p = 0.07); however, statistical significance was not achieved due to sample size. Female age and elective cryopreservation of all 2PN oocytes without a fresh transfer (e.g., hyperresponders) were found to be negatively and positively, respectively, associated with the chance of achieving a live birth.
CONCLUSIONS: The introduction of vitrification has a measurable impact on the efficacy of an IVF program. However, this effect is not large despite the impressively higher cryo-survival rates with vitrification. The "true" net efficacy effect of introducing 2PN vitrification in an IVF program will, in real life, be lower due to patients not having surplus 2PN oocytes available for freezing and later transfer.