Zona pellucida (ZP) protects the oocyte and the developing embryo from infections and holds the initial cells (blastomeres) together while they are still autonomous. As the embryo develops after the stage of morula, (approximately on the 4th day of development, 87-96 hours post fertilization) the blastomeres merge together and the ZP starts thinning. Following implantation the growing embryo has reached the blastocyst stage and performs ZP hatching.
Zona pellucida degenerates and decomposes, to be replaced by the underlying layer of trophoblastic cells and the pregnancy journey begins.
It is suspected that as a result of IVF techniques (multiple maturation of the follicles, medication, vitrification and warming), increased woman’s age etc, the ZP is much more solid and it is harder to hatch naturally.
That is why, in IVF Athens Center in most of IVF cycles we perform artificial opening of the zona with a technique called Assisted Hatching.
The technique is not invasive or harmful, it happens under the inverted microscope using specific laser lenses, and is applied prior the embryo transfer either on day 3 (cleavage stage) or day 5 (blastocyst stage) of development. The same method is also used in PGT cases because the opening of the zona is absolutely necessary to remove the blastomeres for genetic testing.
It has been reported in bibliography, that Assisted Hatching increases pregnancy rates.