The embryo must escape (hatch) from the ZP before implantation may occur.  By 5 or 6 days after fertilization, the embryo has developed into a blastocyst, a 32-64 cell embryo that has expanded forming a hollow sphere.  The blastocyst accomplishes hatching by a combination of enzymatic thinning of the ZP and contraction and re-expansion of the blastocyst.  This causes the ZP to become extremely attenuated and almost invisible, and results in the rupture of the ZP.  The blastocyst escapes and implantation may occur.

            Several factors may inhibit or delay hatching of the blastocyst from the ZP.  These include an unusually thick or dense ZP, poor embryo quality, and delayed growth.  Any of these factors may be inherent in embryos from a select group of patients.  These include older patients (38 years of age and older), patients with some symptoms of early menopause, or patients with repeated IVF cycle failures.   Assisted hatching before embryo transfer may facilitate the natural hatching process in these patients.

            Assisted hatching is performed on embryos prior to embryo transfer.  A high-magnification microscope and delicate micromanipulation instruments are used to create an opening in the ZP.  The blastocyst is able to use this opening to begin its escape from the ZP.  This procedure has improved the outcome for IVF patients in this select group of patients.