High implantation and pregnancy rates with transfer of human hatching day 6 blastocysts.

Although improved outcomes of IVF after transfer of blastocysts has been reported (1, 2), it remains uncertain which blastocyst can develop into a viable fetus. At present, expansion of the blastocyst is the most practical selection criterion. However, if extended culture of embryos produces many expanded-stage blastocysts, it is not easy to decide which blastocysts to transfer (Fig. 1A). In addition, some blastocysts fail to hatch out of the zona, despite evidence that the blastocoelic cavity had expanded and the zona pellucida had thinned (3).