Fresh Versus Vitrified-Warmed Donor Oocytes: Embryological and Clinical Outcomes

This multicenter retrospective observational study compares embryological and clinical outcomes following treatment with fresh versus vitrified-warmed donor oocytes at Bocian Fertility Clinic centers in Poland. The study focuses on a standardized allocation strategy in which recipients are intended to receive six mature (metaphase II, MII) donor oocytes. In the vitrified-warmed group, oocytes are cryopreserved at the MII stage and subsequently warmed for use. Oocytes lost during warming are not routinely replaced, allowing the study to evaluate the consequences of warming-related losses within the initial allocation. The study will assess fertilization, embryo development, usable blastocyst yield, the probability of obtaining an embryo for transfer, and live birth following the first embryo transfer originating from the allocated oocytes. Clinical effectiveness will be evaluated from the initial oocyte allocation, including cycles in which no embryo transfer occurs. The objective is to determine whether fresh and vitrified-warmed donor-oocyte strategies are associated with differences in embryological efficiency and the likelihood of live birth when the initial number of mature allocated oocytes is standardized.

Inclusion Criteria: * Female recipients aged 18 years or older undergoing assisted reproductive treatment using donor oocytes at participating Bocian Fertility Clinic centers in Poland. * Treatment cycles initiated within the prespecified historical study period, with the last eligible cycle occurring on September 30, 2026. * Recipient cycles involving fresh or vitrified-warmed donor oocytes mature at the metaphase II (MII) stage. * An identifiable initial allocation of six mature donor oocytes intended for ICSI, before any warming-related losses. * Availability of laboratory records allowing identification of the donor-oocyte strategy and linkage to the recipient treatment cycle. * Cycles without fertilization, usable embryos, or embryo transfer remain eligible. Exclusion Criteria: * Treatment cycles using autologous rather than donor oocytes. * Cycles involving embryo donation without an identifiable recipient-specific donor-oocyte allocation. * Mixed fresh and vitrified-warmed oocyte allocations within the same index treatment cycle. * Cycles with no verifiable initial six-MII allocation or insufficient information to classify the oocyte strategy. * Duplicate records referring to the same index oocyte allocation.