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.