A comparison of day 5 and day 6 blastocyst transfers

Objective: To compare implantation and pregnancy rates according to the day of embryo transfer (day 5 or 6 after oocyte retrieval) when transfer was postponed until expanded blastocysts developed. Design: Retrospective clinical study. Setting: Private ART center. Patient(s): One-hundred and eighty-three women undergoing blastocyst-stage embryo transfer following in vitro fertilization. Intervention(s): Bipronucleate oocytes were grown for up to 144 hours and subsequently transferred only when at least one embryo attained the expanded blastocyst stage. Main Outcome Measure(s): Implantation and pregnancy rates. Result(s): Blastocysts transferred on day 5 implanted at nearly twice the rate of blastocysts transferred on day 6 (36.3% vs. 19.0%). Pregnancy rates were also almost twice as high among the day 5 transfer patients (59.3% vs. 32.3%). In addition, more blastocysts developed (3.6 vs. 2.4), and more were transferred (2.7 vs. 2.3) to the day 5 transfer patients, although the proportion of expanded blastocysts among the blastocysts that were transferred was the same for the two groups (91.7% vs. 93.6%). Conclusion(s): Embryos that develop to the expanded blastocyst stage and are transferred on day 5 after retrieval are approximately twice as likely to implant compared to those for which expansion and transfer are delayed until day 6. (Fertil Steril(R) 2001;75:1126-30. (C)2001 by American Society for Reproductive Medicine.).