Cryo IVF, frozen embryo transfer (FET), is the process of thawing and transferring a previously cryopreserved embryo rather than a freshly fertilised one. It accounts for 41 percent of all IVF cycles in the UK and consistently achieves clinical outcomes comparable to fresh transfer in experienced programmes.
For the patient, cryo IVF begins after a stimulation cycle in which eggs were collected, fertilised, and surplus embryos were vitrified for storage. In subsequent cycles, the patient does not undergo ovarian stimulation again. Instead, the uterine lining is prepared to receive an embryo, in a natural cycle, a modified natural cycle, or a hormonally programmed cycle using oestrogen and progesterone.
When the endometrium reaches the appropriate thickness and pattern on ultrasound, an embryo is selected from cryogenic storage and the warming procedure begins. The embryologist removes the vitrified embryo from the liquid nitrogen storage tank, warms it rapidly through the warming solution protocol, assesses post-warming survival, and, if the embryo has survived intact, transfers it to the uterus using a soft catheter. The procedure takes a few minutes and requires no anaesthetic.
The quality of the cryopreservation at the point of freezing, the vitrification technique, the storage containers, and the long-term stability of storage conditions in the cryogenic system, directly determines the quality of the embryo available at transfer.
