Should embryos developing to blastocysts on day 7 be cryopreserved and transferred: an analysis of pregnancy and implantation rates

ObjectiveTo compare pregnancy rates (PRs) using blastocysts cryopreserved on day 7 with those cryopreserved on days 5 and 6.DesignRetrospective observational cohort study.SettingInfertility center performing IVF.Patient(s)Eight hundred women with infertility undergoing frozen ET.Intervention(s)Blastocysts cryopreserved on days 5, 6, and 7 after retrieval were thawed and transferred.Main Outcome Measure(s)Ongoing PRs (pregnancy developing appropriately into the second trimester). Thaw survival, implantation rates, and clinical PRs were also calculated.Result(s)A total of 1,406 embryos were thawed with a survival of 90.7% for day 5, 83.7% for day 6, and 78.7% for day 7. Implantation rates were 43.3%, 28.9%, and 28.9%, respectively. Ongoing PRs were 43.9%, 32.9%, and 26.7%, respectively.Conclusion(s)Blastocysts cryopreserved on day 7 have a lower, but clinically important potential. Embryos that do not achieve blastocyst stage on day 6 should not be universally discarded, but should be observed in culture 1 more day as 27% may result in an ongoing pregnancy.