Under normal circumstances the protective layer that surrounds the embryo known as the zona pellucida undergoes gradual thinning due to the expansion of the growing embryo and the increased pressure causes the zona to break and the embryo hatches and then implants in the uterus around day 5 or day 6.

In some specific couples , the embryos have very thick zona which may be the cause of implantation failure. Assisted Hatching is the method using micromanipulation techniques such as mechanical , chemical or laser to create a small hole at the zona which may help the embryo escaping from its shell.