Shorter protocols for vitrification and post-warming dilution of human oocytes and embryos: a narrative review

About 20 years ago, vitrification was introduced to IVF laboratories, quickly surpassing the traditional slow-freezing methods used until then. As vitrification improved survival and pregnancy rates, it opened new opportunities for treatments, such as elective egg freezing and deferred embryo transfer. Vitrification has now become the preferred cryopreservation method worldwide, and current efforts aim to enhance its efficiency. Shortening equilibration and rehydration in vitrification and warming protocols is desirable for several reasons. First, it would simplify procedures, making competency and consistency easier to achieve. Second, it would reduce the time of exposure to suboptimal temperature and osmolality while ensuring optimum results. Finally, shortening times would help mitigate potential genetic, epigenetic and other toxic effects associated with prolonged exposure to cryoprotective agents. This review aims to provide background information to understand the latest updates and evolution of cryopreservation in assisted reproductive technology, from its origins in slow freezing to the introduction of new, efficient equilibration and rehydration protocols.