embryos having undergone cryopreservation implant at the same rate as equivalent fresh counterparts. The outcome from using cryopreserved embryos has uniformly been positive with no increase in birth defects or development abnormalities, also between fresh versus frozen eggs used for intracytoplasmic sperm injection (ICSI). In fact, pregnancy rates are increased following frozen embryo transfer, and perinatal outcomes are less affected, compared to embryo transfer in the same cycle as ovarian hyperstimulation was performed. The endometrium is believed to not be optimally prepared for implantation following ovarian hyperstimulation, and therefore frozen embryo transfer avails for a separate cycle to focus on optimizing the chances of successful implantation. Children born from vitrified blastocysts have significantly higher birthweight than those born from non-frozen blastocysts. For early cleavage embryos, frozen ones appear to have at least as good obstetric outcome, measured as preterm birth and low birthweight for children born after cryopreservation as compared with children born after fresh cycles. Oocyte age, survival proportion, and number of transferred embryos are predictors of pregnancy outcome. Pregnancies have been reported from embryos stored for 27 years. A study of more than 11,000 cryopreserved human embryos showed no significant effect of storage time on post-thaw survival for IVF or oocyte donation cycles, or for embryos frozen at the pronuclear or cleavage stages. In addition, the duration of storage had no significant effect on clinical pregnancy, miscarriage, implantation, or live birth rate, whether from IVF or oocyte donation cycles. A study in France between 1999 and 2011 came to the result that embryo freezing before administration of gonadotoxic chemotherapy agents to females caused a delay of treatment in 34% of cases, and a live birth in 27% of surviving cases who wanted to become pregnant, with the follow-up time varying between 1 and 13 years. Legislation From 1 October 2009, human embryos are allowed to be stored for 10 years in the UK, according to the Human Fertilisation and Embryology Act 2008. History The cryopreservation of embryos was first successfully attempted in 1984 in the case of Zoe Leyland, the first baby to be born from a frozen embryo. In Zoe's case, the embryo had been frozen for two months, but since the inception of the practice of cryopreservation after successful IVF, embryos have successfully survived in cryopreservation extensively longer periods of time, spanning even decades. The long-term implications of freezing embryos are demonstrated in the case of