In their first days of life, the embryo grows within an envelope or shell called the area pellucida (the same that wrapped around the oocyte before being fertilized). This wrapper isolates the embryo helping to maintain the cohesion of the blastómeras and, in turn, favoring their transit through fallopian tube. During this period, which lasts between five and seven days, the embryo grows and develops independent until you reach the uterus, where reach the blastocyst stage about the fifth day after fertilization. Arriving at this stage, and as cell divisions continue, the embryo forms a cavity of liquid (blastocoele) which will be progressively increased volume at the same time that the envelope (area pellucida) is tuned more and more until it reaches the time in which breaks, allowing the exit of the embryo. This process in which the embryo is released from his deck we call hatching, although it is also frequently used the anglicism hatching to describe this phenomenon. Although the process is favoured by the influence of the endometrium, the embryo holds itself to complete this process. Of factor it is frequent that some embryos in culture hatch before being transferred or frozen.
After they hatch, the embryo may interact directly with the endometrium giving rise to the implementation process and the pregnancy itself. Pellucid zone free-blastocyst after hatching

There are cases where, by the morphological characteristics of the embryo, the hatching process may be compromised the embryo to prevent his release of the pellucida zone. In these cases, the embryo will grow within your wrapper for a day or two more before entering apoptosis and die, so never take place pregnancy despite having achieved the fertilization.
Among the factors that may affect the hatching process are the following:
-Excessive thickening of the pellucida zone (> 15 µm). This thickening can be accentuated by the treatment of ovarian stimulation with high doses of gonadotropins or when the FSH levels are high.
-Structural and biochemical alterations in the area pellucida may be reflected in a darkening of this (caramel colour) Oocyte with thickened zona pellucide
-High fragmentation of the embryo. At the time of fragmentation, the resulting fragmentsremain on the periphery of the embryo (between this and the wrapping) and can affect the area pellucida thinning mechanism. In turn, when there is an excessive fragmentation of the embryo cell volume is less and its expansion will be less and may come to be insufficient to break the area pellucida.