During initial stages of development, the embryo hatches the outer layer of embryo which is called “zona” in a particular stage and attaches to the walls of the uterus. Some patients have a thicker zona. This is sometimes due to age or sometimes arises for no reason. Zona may become rigid or thick depending on the process following thawing of the frozen embryos. In such cases, the embryo is unable to hatch out the zona thus has difficulty in attaching to the walls of the uterus.
In this case, the zone is made thin mechanically or using a laser. Making the zona thin using any of these methods is called assisted hatching.