In 1978, the world’s first IVF case was born in the United Kingdom, and for 30 years, IVF, or in vitro fertilization-embryo transfer technology, has enabled countless infertile couples around the world to realize their dream of having children. “The embryo freezing technique is one of the most important techniques of IVF. Due to the use of ovulation-promoting drugs, several, a dozen or even dozens of embryos can be obtained in a single IVF treatment cycle, and up to three embryos can be transferred into the uterus in a single embryo transfer, while the remaining high-quality embryos are recommended to be frozen for preservation. Since 1983, the world’s first “frozen baby” was born in Australia, and as of 2008, approximately 350,000 to 500,000 babies have been born in the world using the frozen embryo technique. After more than 200 years of development of the amazing cryobiology, the embryo freezing technology has become more and more perfect, and the simpler and better preserved “vitrification” technology has been adopted by more and more assisted reproduction centers worldwide. It has been adopted by more and more assisted reproduction centers worldwide. It is important to note here that only “good quality embryos” are worth freezing, as early quality embryos should contain fewer cell fragments and evenly sized ovoid spheres, while late stage embryos – blastocysts – should have a relatively full inner cell mass and an intact trophoblast layer. Embryos that do not meet the criteria have limited developmental capacity of their own, and after the freeze-preserve-thaw process, their likelihood of further development is even lower, and doctors routinely advise patient couples to abandon such embryos for destruction. How long can embryos be frozen and preserved? Is the survival rate worse the longer the freezing period? Do frozen embryo babies have more birth defects than regular IVF and natural babies? These should be the main concerns of IVF parents, and we will answer them one by one with a lot of statistics from home and abroad. Regardless of the freezing technique, the final embryos will be stored in liquid nitrogen (-196°C), and only a very small number of centers in the world store embryos in liquid nitrogen vapor (-180-185°C). In liquid nitrogen, the cellular enzymes are almost completely inhibited, which means that the cellular processes are at a “standstill” and the only damage is caused by the genetic damage caused by external physical radiation. Experts have demonstrated that irradiation of mouse embryos preserved in liquid nitrogen with a dose of radiation equivalent to 2000 years in nature does not affect their viability and vitality, nor does it lead to an increase in the rate of malformations. This means that in theory, embryos can be preserved for more than 2000 years in a stable liquid nitrogen environment and under natural radiation! Of course, this is far from being proven in practice, nor is it possible to reach this point; in 2006, a baby embryo was born in Spain that had been preserved in liquid nitrogen for 13 years; and this record was set in 2010, when a baby embryo was born in the United States that had been preserved in liquid nitrogen for 13.5 years! Therefore, in practical terms, embryos can be stored in liquid nitrogen for 5-10 years, but the extended freezing time comes with many medical and socio-ethical problems, and the embryo banks of large fertility centers in China are currently full of hard-earned but “unusable” embryos of couples who have successfully given birth. As mothers age, the chances of conception decline and the existence of these embryos becomes less and less relevant. In the UK, people can keep their frozen embryos for 5 years, and if they want to extend the period, they need to apply to the government; in Spain, there is no regulation on the time limit for embryo freezing and preservation; in China, there is no regulation yet, but it is believed that there will be a relatively perfect policy in the near future. Research and analysis The duration of cryopreservation has no effect on embryo survival and fertilization rate. In a study of 11768 frozen embryo transfer cycles, it was found that there was no difference in embryo survival rate, embryo implantation rate, clinical pregnancy rate, miscarriage rate, and live birth rate when comparing embryo cryopreservation for 1-3 months, 3 months-1 year, 1-2 years, 2-3 years, and more than 3 years. In another study comparing embryos preserved for 6 months with those preserved for 2-9 years, there was also no difference in embryo survival rate and embryo implantation rate. It is evident that the length of cryopreservation does not affect the ability of embryos to continue development. Sufficient Evidence For the health of babies born from frozen embryos, there is also sufficient evidence for the safety of embryo freezing applications. In a large retrospective study, the incidence of birth defects was instead lower in frozen embryo transfer cycles (3502 cases) compared to fresh transfer cycles (4039 cases). In a perinatal survey, frozen embryos were found to have a relatively lower rate of preterm delivery, a lower rate of low birth weight, and a lower incidence of fetal growth restriction compared to fresh embryos after transfer. This is certainly related to the fact that the maternal status of the frozen embryo transfer cycle may be superior to that of the post-ovulation (fresh embryo cycle). A study that followed children born with frozen embryos between the ages of 1 and 9 years did not find significant defects in height, weight, development, or IQ in these children. A large body of clinical data confirms the safety of frozen embryos, which can be stored stably for 5-10 years, and that the length of cryopreservation does not affect the ability of the embryos to continue to develop. The birth weight and developmental status of babies born with frozen embryos, as well as the physical and intellectual development of children later in life, are within normal limits. We are looking forward to further development of embryo culture technology and freezing technology to provide a better guarantee of human fertility preservation