Oocytes and embryos are surrounded by a layer of connective tissue-like sheath – the zona pellucida (specifically a thick, transparent membrane). To be capable of nidation, an embryo in the blastocyst stage usually hatches from this sheath.
Under certain circumstances, the zona pellucida can become hardened, making it challenging or too complicated for hatching of the embryo to be possible. For instance, this can be the case for embryos that have emerged from cryopreserved oocytes. The age of the female also appears to play a role with regard to the duress or hardness of the zona pellucida.
It is possible to facilitate hatching by a laser-like induction of a weak spot (‘predetermined breaking point’) in the zona pellucida.
This increases the likelihood of the hatching of the embryo.
Only a hatched embryo is able to implant itself in the uterus.