Embryo freezing involves the cryopreservation, or rapid freezing, of a blastocyst around day 5 or 6, a process known as vitrification. Vitrification eliminates the formation of damaging ice crystal, leading to high recovery rates when the embryos are thawed. We aim to freeze embryos at the blastocyst stage as it has been shown that they have the best recovery and live birth rates.
The selected embryos are placed in a cryoprotectant and inserted into straws labelled with unique identifying information. The straws are then stored in liquid nitrogen dewars and kept in a suspended state at -196 degrees Centigrade in our state-of-the ART laboratory.
A patient having an IVF or ICSI cycle will have a single embryo transfer and any surplus embryos that are of good quality can be selected for freezing. This offers the possibility of a frozen embryo transfer in either a natural or hormone replacement cycle.
Can all embryos be frozen and would embryo freezing damage them?
Unfortunately, not all embryos will be suitable for cryopreservation. Culturing embryos to the blastocyst stage gives us the opportunity to select the best quality embryos, with blastocysts having the greatest potential of implantation. Only good quality blastocysts can be frozen, i.e. grade A or B, as they have the highest recovery rates following the freezing and warming process.
Currently, around 95% of frozen embryos recover well following warming. There is no evidence that neither freezing nor prolonged storage impairs the ability of an embryo to implant and develop into a healthy baby. A number of live births have been reported worldwide with embryos that were frozen for more than 10 years.
The UK HFEA latest trends and figures for 2019 reported an increase of frozen embryo transfer cycles by 86% in the period 2014-2019. The success rates of frozen embryo transfer cycles are now comparable to fresh embryo transfer cycles.
How long can embryos be stored?
Embryos can be stored for up to 10 years. As embryos are created from sperm and eggs, we would require both gamete providers (unless using donor sperm) to have individually consented to the terms under which an embryo can be cryopreserved and stored.
If a medical condition is involved that could result in impairing your fertility in the future, such as cancer treatment or hormone replacement therapy for gender reassignment, we can extend the storage period up to 55 years. To enable us to do so, we would need your written consent and a medical letter from your GP/consultant verifying the medical condition. In addition, embryos can only be used if both gamete providers have provided us with a written consent to the use that is being proposed.
What happens to my frozen embryos if I do not want to use them?
You can withdraw your consent at any point during the 10-year (or 55-year) period, should you not want to keep your embryos in storage any longer. Please contact us and we will provide you with the relevant consent forms that you and your partner (unless using donor sperm) need to complete before we can terminate the embryo storage.
