How does vitrification affect oocyte viability in oocyte donation cycles? A prospective study to compare outcomes achieved with fresh versus vitrified sibling oocytes.

STUDY QUESTION: How does vitrification affect oocyte viability? SUMMARY ANSWER: Vitrification does not affect oocyte viability in oocyte donation cycles. WHAT IS KNOWN ALREADY: Oocyte vitrification is performed routinely and successfully in IVF and oocyte donation programs. STUDY DESIGN, SIZE, DURATION: This is a prospective study performed between June 2009 and February 2012 to compare ongoing pregnancy rates and other indices of viability between fresh and vitrified oocytes. A total of 99 donations with more than 16 oocytes (MII) in which oocytes were allocated both to a synchronous recipient (fresh oocytes) and to an asynchronous recipient (vitrified oocytes) were included. PARTICIPANTS/MATERIALS, SETTING,
METHODS: The participants were consenting couples (donors and recipients) from the oocyte donation program. On the day of retrieval, the oocytes allocated to the synchronous recipient were inseminated and those allocated for banking were denuded of cumulus and vitrified. Vitrified oocytes were microinjected with spermatozoa 2 h after warming. Embryo transfer was performed on Day 2 of development in both groups, and the remaining embryos were cryopreserved on Day 3. Clinical pregnancy was defined by a positive fetal heartbeat at 6 weeks. MAIN RESULTS AND ROLE OF CHANCE: A total of 989 oocytes were warmed and 85.6% survived. No significant differences were observed between fresh and vitrified oocytes: fertilization rate (80.7 versus 78.2%), ongoing embryo rate (71.0 versus 68.2%) or good-quality embryo rate (54.1 versus 49.8%). The mean number of embryos transferred was similar in both groups (1.82 ± 0.44 versus 1.90 ± 0.34). The implantation rate (33.3 versus 34.0%) and the multiple pregnancy rate (27.7 versus 20.8) were also similar between both groups (P > 0.05). The live birth rate per cycle was 38.4% in the recipients of fresh oocytes and 43.4% in the recipients of vitrified oocytes (P > 0.05). Eighty five frozen embryo transfers were also evaluated. Comparing embryos from fresh and vitrified oocytes there were no significant differences in the embryo survival rate (70.1 versus 65.8%), clinical pregnancy rate (40.8 versus 33.3%) or implantation rate (21.8 versus 26.8%). LIMITATIONS, REASONS FOR CAUTION: The oocytes were donated by healthy, young women (≤35 years) and these results cannot be extrapolated to other populations. WIDER IMPLICATIONS OF THE
FINDINGS: Outcomes obtained with vitrified oocytes are as good as with fresh oocytes and the use of vitrification can be extended to new applications, e.g. accumulation of oocytes from successive stimulations for preimplantation genetic diagnosis, for patients at risk of ovarian hyperstimulation syndrome or in patients needing to preserve their fertility. STUDY FUNDING/COMPETING INTEREST(S): This work was done under the auspices of the Càtedra d'Investigació en Obstetrícia i Ginecologia of the Universitat Autònoma de Barcelona.