Embryo Freezing (Vitrification) Explained
Embryo freezing stores suitable embryos created during IVF that are not transferred, so they can be used later. The method used today is a rapid freezing technique called vitrification.
This article explains how it is performed, when it is chosen, and how it differs from egg freezing.
What is vitrification?
Vitrification cools the embryo so rapidly that it passes into a glass-like state, which prevents ice crystals forming inside the cells.
Because ice crystals can damage cell structure, this rapid method has replaced the older slow-freezing techniques. Embryos are stored in liquid nitrogen on labelled, individually recorded carriers.
When is it used?
Embryo freezing comes into play in several situations:
- Suitable embryos remaining after a transfer
- Freezing all embryos and postponing transfer (freeze-all)
- Moving transfer to a safer point because of OHSS risk
- Waiting for the results of genetic testing (PGT)
- The uterine environment not yet being suitable for transfer
- Preserving fertility ahead of a medical treatment
How are embryos thawed?
When transfer is planned, the embryo is warmed under controlled conditions and assessed in the laboratory, where its survival and structure are checked.
A suitable embryo is then transferred in a prepared cycle. See our page on frozen embryo transfer for the full process.
How long can embryos be stored?
Storage duration is set by regulation and requires written consent that is renewed at defined intervals. Your centre will explain the storage period and the renewal schedule.
Storage depends on the couple's joint decision and on the rules in force. It is worth clarifying this with your doctor before starting treatment.
Is embryo freezing safe?
Vitrification is an established laboratory method used routinely today, and the large majority of embryos survive the warming process.
Outcomes depend on the quality of the embryo before freezing and on laboratory conditions, so they vary from person to person.
How it differs from egg freezing
Egg freezing stores unfertilised eggs and needs no sperm at that stage. Embryo freezing stores an embryo that has already been fertilised and whose development has been observed.
Which approach suits you depends on your circumstances and your plans.
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Sources and references
This article is supported by the following independent, authoritative sources: