With cryopreservation techniques surplus embryos in an IVF cycle may be stored for future use. If the IVF cycle, where fresh embryos are transferred, the cryopreserved embryos can be thawed and transferred in a subsequent cycle. If the fresh IVF cycle is successful then the embryos can be stored, within the time limits specified by Law, until the couple decides to have another child. According to Greek Law 3305/2005 on assisted reproduction cryopreserved embryos can be stored for five years with the option of extending the storage period to an additional five years.
The success rates of IVF cycles with frozen/thawed embryo transfers are similar to those with fresh embryo transfers. Studies have shown that the cryopreservation techniques are safe and do not affect embryo quality. It is encouraging that children born from frozen/thawed IVF cycles are healthy and normal. Many studies have shown that the chances of developmental or other abnormalities are not increased in children born from frozen/thawed cycles when compared to those from fresh cycles.
Slow freezing
The embryo is protected during slow freezing with the addition of cryoprotectants that inhibit the creation of salts and crystals during the freezing process that may harm embryonic cells. The cryopreserved embryos are stored in liquid nitrogen. The slow freezing process lasts a few hours and is monitored with the suitable equipment, whereas the thawing process lasts 30 to 45 minutes. It has been observed than embryos that had been in cryo-storage for 13 years have been thawed successfully, and clinical pregnancies have been achieved with embryos that were stored for nine years. The most common time frame for cryo-storage and successful pregnancy is five years. Embryos that are cryopreserved at the pronuclear stage with slow freezing have better chances of survival, development and implantation than those of a higher cell number.
Vitrification
Embryos at the blastocyst stage (around day 5 post fertilisation) are larger embryos and need special treatment for successful cryopreservation. In recent years the vitrification method has been developed that can be used for oocyte and embryo freezing at any stage of development. This methods uses suitable cryoprotective media of very high concentration in which the embryos or oocytes are cultured for a small amount of time (few minutes). The freezing rate is much quicker than that of slow freezing. In this way the water inside the embryonic cells does not form “lethal” crystals as it freezes instantly. Vitrification is highly recommended for the cryopreservation of blastocysts as they are very hydrated embryos and therefore more vulnerable in the formation of water crystals.
Embryo cryopreservation and ovarian hyperstimulation syndrome
The ovarian hyperstimulation syndrome occurs rarely during the IVF procedure. It is a serious condition that may arise during the follicular phase or the beginning of pregnancy. If a pregnancy is achieved in this IVF cycle the syndrome effects may be more serious and last longer. For this reason it is important to avoid a pregnancy if signs of ovarian hyperstimulation are observed. The embryos can be cryopreserved and transferred in a subsequent cycle and after the hyperstimulation symptoms have been treated.
In coclusion, embryo cryopreservation is very important in assisted reproduction as it extents the chance of positive outcome when fresh cycles are unsuccessful or when the couple wishes to have another child following a successful IVF cycle.