When an embryo in a natural pregnancy reaches the blastocyst stage of development, the zona starts to thin. The blastocyst repeatedly contracts and expands, changing the flexibility of the zona, which promotes the thinning process. The zona tears when it has been sufficiently thinned, allowing the embryo to hatch. Failure to hatch an embryo prevents it from implanting. Natural hatchings in human blastocyst have been found to frequently take place close to the inner cell mass, according to numerous investigations. Assistance with hatching
