LAH is used in patients with poor prognosis to help with the implantation of the embryo. The embryo in its early stage stays within a membrane that is known as Zona Pellucida. The embryo must first break through the zona pellucida to implant successfully in the uterine lining. In Assisted hatching, an embryologist does the same. He uses micromanipulation techniques under a microscope and makes a small hole in the zona pellucida, which helps the embryo to implant in the uterine lining. Some embryos seem to have a thicker shell that may diminish their ability to hatch and reduce the likelihood that they will implant. This may be due to the age of the woman or other unknown reasons.
Hatching of the embryo is necessary to achieve pregnancy. It has been reported that up to 75% of normal embryos may never hatch. Laboratory procedures involved in IVF may add to the hardening of the zona, as the embryo is not continually exposed to the enzymes present in the natural environment of the Fallopian tubes. There is also evidence to suggest that the process of freezing and thawing embryos may cause the zona to harden and using assisted hatching can be beneficial.