Factors affecting thawed oocyte viability suggest a customized policy of embryo transfer

ObjectiveTo identify factors that might affect the clinical outcome of oocyte slow freezing.DesignRetrospective study.SettingReproductive Medicine Unit, Italian Society for the Study of Reproductive Medicine, Bologna, Italy.Patient(s)Patients with spare metaphase II cryopreserved oocytes performing 371 thawing cycles.Intervention(s)Oocytes were cryopreserved by slow freezing <40 hours after hCG administration (group A) and ≥40 hours after hCG administration (group B). Thawed oocytes were inseminated by intracytoplasmic sperm injection.Main Outcome Measure(s)Clinical pregnancy, implantation, abortion, and delivery rates.Result(s)Clinical pregnancy rate per thawed cycle (PR) and implantation rate (IR) were significantly higher in group A compared with group B both in young (PR: 25% vs. 9.6%; IR: 18.9% vs. 8.8%) and in older patients (PR: 25% vs. 10.1%; IR: 17.5% vs. 6.7%). In the young patient subgroup, clinical pregnancy and implantation rates with three transferred embryos were higher in group A vs. group B (PR: 72.7% vs. 25%, and IR: 36.4% vs. 12.5%, respectively). This difference was not found in the subgroup of older patients.Conclusion(s)The timing at which oocyte cryopreservation is performed and the number of transferred embryos play a key role in the clinical outcome. The suggested cut-off time for cryopreservation is between 39 and 40 hours after hCG administration.