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When a conventional IVF or ICSI's cycle is performed, surplus good quality embryos can be left over after embryo transfer. If this were the case, these surplus embryos are cryopreserved and can be used later on in a cryotransfer cycle which does not require ovarian stimulation.
Embryos are cryopreserved in liquid nitrogen and stored in cryogenic tanks. According to the current Spanish legislation of assisted reproduction, embryos can be frozen during the whole reproductive life of the woman.
Though the rates of gestation of defrosted embryos are slightly lower, the process is much simpler and less expensive.