['To assess on which day to cryopreserve and transfer thawed embryos in good-responder patients by comparing the cycle outcomes of day 3 transfers vs blastocysts formed through extended culture before or after cryopreservation.', 'Retrospective clinical study.', 'Private IVF center.', 'Frozen-thawed cycles (n = 2,531) who had ETs at day 3, 5, and 6 and post-thawed extended culture of day 3 until day 5 or 6.', 'None.', 'Primary outcomes were implantation and delivery rates. Secondary outcomes were clinical pregnancy and miscarriage rates.', 'In thawing cycles, embryos developing to blastocysts on day 5 through extended culture before or after cryopreservation yielded higher rates of implantation (51.1% and 51.3%, respectively), clinical pregnancy (69.9% and 62.2%, respectively), and delivery per thawing cycle (56.7% and 51%, respectively) accompanied by lower miscarriage rates (15.2%, 16.4%, respectively) compared with day 3 transfers (28.3%, 55.3%, 42.5%, 20.1%, respectively). Late-developing embryos formed before or after cryopreservation resulted in compromised implantation (44.7% and 44.2%, respectively), clinical pregnancy (59.9% and 45.9%, respectively), delivery per thawing cycle (42.8% and 32.4%, respectively) and higher miscarriage rates (25.7% and 23.5%, respectively) than day 5 embryos.'] Is cryopreservation of blastocysts the most feasible strategy in good responder patients?