Prior to implantation at the endometrium lining, the developing embryo must hatch out from zona pellucida (egg shell). Some embryos seem to have a thicker and harder shell that may decrease their ability to implant. This may due to the age of the woman of other unknown reasons.
In order to assist the embryo to hatch, a small opening is created at the zona pellucida using either laser, acid or enzyme technique. This process is called assisted hatching.