Embryo cryopreservation is the process of freezing human embryos at ultra-low temperatures to preserve their viability for future use in IVF treatment. It is one of the most clinically transformative techniques in modern reproductive medicine, allowing surplus embryos from a stimulation cycle to be stored for future frozen embryo transfer cycles, significantly improving cumulative live birth rates without repeated ovarian stimulation.
The process begins after embryos have been cultured to the blastocyst stage, typically five to six days after fertilisation. The embryologist selects embryos of sufficient quality and exposes them to cryoprotective agents in a stepwise protocol. Permeating agents such as ethylene glycol and DMSO enter the cell and replace intracellular water, while non-permeating agents such as sucrose draw additional water out osmotically. This dehydration step is essential: the less intracellular water remains, the lower the risk of ice crystal formation during the freeze, which would otherwise puncture cell membranes and destroy viability.
The dehydrated embryo is then loaded onto a vitrification device in an extremely small volume of solution and plunged directly into liquid nitrogen at -196 degrees Celsius. The cooling rate during vitrification exceeds 15,000 degrees Celsius per minute, converting the intracellular fluid from liquid to a glass-like amorphous solid state without crystal formation. Post-warming survival rates for vitrified blastocysts now consistently exceed 95 percent in experienced IVF laboratories, compared to 60 to 70 percent with earlier slow-cooling protocols.
Once vitrified, embryos are transferred to a liquid nitrogen storage tank where they remain at -196 degrees Celsius until the patient is ready for a frozen embryo transfer cycle. At this temperature, all biological activity is completely halted and embryos remain viable indefinitely under correct storage conditions. A study published in Human Reproduction in 2020 examining embryos stored for between one and nine years found no statistically significant difference in live birth rates based on duration of storage, confirming that storage conditions rather than storage duration determine long-term viability.
Warming reverses the vitrification process by rapidly raising the temperature to prevent ice recrystallisation. Warming solutions progressively dilute the cryoprotective agents and rehydrate the cells. The warming step is as technically demanding as the freeze — deviation from the protocol at this stage can destroy embryos that survived both the freeze and years of storage in perfect condition.
In the UK, embryo cryopreservation is regulated by the HFEA. The HFEA’s Fertility Trends report for 2022 showed that frozen embryo transfers now account for 41 percent of all IVF treatment cycles in the UK, making cryopreservation a central pillar of modern IVF practice rather than a supplementary technique.
Source: HFEA Fertility Trends and Figures 2022. Human Reproduction, Vol. 35, No. 1, 2020.
Further reading: Cryopreservation Guide | Vitrification Guide | Liquid Nitrogen Tanks
