Clinical efficiency of oocyte and embryo cryopreservation.

The assessment of the standard for success of in vitro fertilization (IVF) treatment is an arduous task. Clinical efficiency and safety of a given procedure should represent fundamental tools for objective comparison. However, differences in patient populations, laboratory protocols, and expression of clinical data make the analysis of different studies and strategies very difficult. Formulation of the standard for success through the cumulative delivery rate per cycle of stimulation is a very attractive option because it includes the essential contribution of frozen embryos, which can represent 30-40% of all deliveries, while taking into account the need to minimize the proportion of pharmacological and surgical treatments. Embryo cryopreservation may be applied at different postinsemination stages. Larger and more detailed sets of data are available for day 2 embryo freezing, which allows cumulative delivery rates of 50-60% in good prognosis patients. In the last few years, novel freezing methods have improved the overall efficiency of oocyte cryopreservation. Especially in contexts afflicted by legal restrictions to embryo cryo- preservation, this form of preservation has started to have an impact on the IVF standard of success, generating cumulative pregnancy rates approaching 50%. Despite having been applied systematically by some IVF programs for only a few years, oocyte freezing already competes in efficiency with pronuclear-stage cryopreservation, and it does not appear unrealistic to predict that in the future it will challenge the dominance of embryo cryopreservation as the preferred form of conservation.