['To determine how the quality of blastocysts formed on day 5/6 of extended culture compares with their morphology on day 3.', 'Retrospective observational study of IVF laboratory records.', 'Private assisted reproduction clinic.', '101 IVF cycles in which 5 to 25 embryos were produced. The average maternal age was 33.1 years.', 'Embryos were individually cultured in vitro in sequential media for an extended time to enable use of blastocysts for fresh transfer or cryopreservation.', 'Comparison of embryo quality for putative ET or cryopreservation on day 3 with quality of embryos used for actual ET and cryopreservation on day 5/6.', 'Of 1,263 cleaving embryos, 559 were judged to have been suitable for use on day 3; 355 would have been used for ET (average per ET, 3.5) and 204 would have been frozen (equivalent to 44% utilization). In actuality, 471 blastocysts were used on day 5/6, of which 234 were transferred (average per ET, 2.3), and 237 were frozen (equivalent to 37% utilization). Only 48% embryos that would have been chosen for ET and/or cryopreservation on day 3 were eventually used in such a manner at the blastocyst stage. Historically, the rate of viable pregnancy from day 3 transfers was 30.5% per transfer; this rate increased to 45% with routine day 5/6 transfers.'] Is day 3 morphology a poor predictor of blastocyst quality in extended culture?