Embryo freezing

For single embryo transfer (SET), it is very important that the best quality embryo is selected to be transferred.

It is equally important that, if any good quality embryos remain, they are frozen for use later. This may avoid the need for repeated drug stimulation, egg retrieval, sperm collection and fertilisation.

What happens to the embryos during freezing and thawing?

When embryos are frozen, every effort is made to ensure they are kept safely and securely:

• embryos are frozen and then stored at very low temperatures in tanks of liquid nitrogen.
• the embryos are stored in sealed, labelled straws in locked and alarmed storage containers.
• the freezing and thawing of embryos is carried out very carefully and a liquid called a cryoprotectant is used to protect the embryos.

Despite the precautions that are taken, not all embryos survive the freezing and thawing process. Occasionally, there may be no embryos that survive, although the chance of this happening is reduced by the embryologist selecting only good quality embryos for freezing.

How do embryologists decide which embryos to freeze?

Embryologists usually assess embryos on the day of transfer and decide whether the embryos are suitable for freezing. The embryos are normally assessed by a grading system. Embryos that are at the correct stage of their development and are of good quality may be frozen.

Clinics’ policies on freezing embryos

Clinics have their own policies on freezing embryos. Some clinics may freeze a single good quality embryo, other clinics may have a policy that recommends you have a certain number of good quality embryos to freeze to give you the best chance of success.

If you are opting to have a single embryo transfer, you should discuss your clinic’s freezing policy with your clinician.