Relevance of Assisted Hatching with Blastocyst Stage Transfer

When a blastocyst expands with the growth of the blastocoel, it stretches and thins the zona pellucida (ZP), ultimately rupturing the ZP allowing the embryo to hatch free ready for implantation. In the event this does not occur, then an otherwise potentially viable embryo may not implant. Assisted hatching (AH) has been developed to overcome this, but has been applied mostly to day-2/3 embryos (Cohen et al, 1992). More recently it has been reported that total zona removal with pronase can be successfully undertaken with human blastocysts (Fong et al, 1998), to enable “naked” fully-hatched blastocysts to be transferred to the uterus ready for immediate implantation. By taking a less aggressive approach with acidified Tyrode’s medium, it is equally possible to drill a large hole in the ZP of a blastocyst, without the risks associated with global exposure of the embryo to pronase. Initial retrospective comparison of AH applied in this manner to the “slower” day-6 blastocysts suggests a distinct