Embryo vitrification normally occurs after a cycle of in vitro fertilization, where the embryo with the highest pregnancy potential was transferred and additional good quality embryos were obtained. In these cases, the potential embryos can be vitrified in order to preserve their development capacity so they can be transferred in a later cycle.
There are also other cases in which patients opt for embryo vitrification, such as when pregnancy is not achieved and they want to increase their chances for that same IVF cycle.