The oocyte and preimplantation embryo is surrounded by a gel-like coating, the zona pellucida, which offers protection during fertilization and transport through the reproductive tract. Through the early stages of development, in vivo and in vitro, the zona will thin as the embryo grows and expands. This thinning will eventually result in a breach of the zona whereby the embryo can escape, as the zona’s protective role is unnecessary and actually inhibitory to implantation at this time.
It is thought that in certain classifications of patients, the embryo fails to hatch from the zona pellucida, therefore, implantation is not established. Assisted embryo hatching (AEH) is a laboratory procedure performed prior to embryo transfer. AEH is an attempt to assist in the natural hatching process by creating a small, chemically induced breach in the zona. We routinely perform AEH on women 38 and over, during frozen embryo transfers, after a previous failed attempt, if the zonae fail to thin in culture, and if the patient’s baseline FSH is elevated.