The blastocoele is a fluid-filled intercellular cavity which increases its volume during blastocyst expansion. This process eventually leads to hatching, allowing thus the blastocyst to enter into contact with the uterine endometrium and to implant.
|Small||the volume of the blastocoele is less than 50% of the total volume|
|Medium||the volume of the blastocoele is greater than 50% of the total volume|
|Large||the blastocoele occupies the whole volume inside the zona pellucida and has started expansion, leading to a thinning of the ZP|