Transfer of nonassisted hatched and hatching human blastocysts after in vitro fertilization

Objective: To determine the feasibility of performing blastocyst transfer 6 days after oocyte insemination. Design: Retrospective clinical study. Setting: University-based IVF center. Patient(s): All cases of IVF over a 1-year span of time (June 1998 -1999) in which seven 2PN embryos were available for transfer. Intervention(s): None. Main Outcome Measure(s): Implantation, pregnancy, and multiple pregnancy rates. Result(s): Transfer of blastocysts on days 5 and 6 resulted in implantation rates of 69% and 33% (P=0.0006), clinical pregnancy rates of 89% and 59% (P=0.05), and multiple pregnancy rates of 39% and 10% (P=0.03), respectively. In cases in which blastocysts were spontaneously hatching or hatched on day 6 (9% of embryos), implantation and pregnancy rates were 52% and 80%, respectively. Embryos were successfully frozen in the hatched or hatching state with resultant clinical pregnancies. Conclusion(s): Transfer of embryos can be delayed to day 6 after oocyte insemination at which time a small percentage of embryos will hatch. Hatching of embryos by day 6 is a favorable prognostic factor for IVF outcome. Embryos that fail to hatch by day 6 may have a lower implantation potential. Difficulty with hatching embryos sticking to the transfer catheter was not encountered. Furthermore, hatching and hatched embryos can be frozen and with subsequent transfer result in pregnancies. (Fertil Steril(R) 2000;74:163-5. (C) 2000 by American Society for Reproductive Medicine).