What is Embryo Freezing?

Freezing good quality embryos offers a chance of conceiving after an unsuccessful fresh embryo transfer, or even several years following a successful transfer and pregnancy. Some patients have had babies more than ten years after their embryos were originally created and frozen. Under controlled conditions, embryos are cooled and stored at extremely cold temperatures (-196°C) which suspends their development. Once frozen, embryos can remain in liquid nitrogen for years without deterioration before being thawed and, if they survive and develop well, transferred to the woman’s uterus during a frozen embryo transfer [FET].

Using thawed frozen embryos gives the possibility of a brother or sister for a patient’s baby from a single cycle of IVF or ICSI treatment.

Who may benefit from embryo freezing?

  • Couples who achieve many good quality 'spare' embryos, following embryo transfer, which can be stored for possible future attempts to conceive patients who have had an egg collection for IVF or ICSI treatment, who have been advised not to go ahead with a fresh embryo transfer will have their embryos frozen for use in treatment at a later date.
  • A woman in a relationship, about to undergo medical treatment that may make her infertile in the future can have embryos created and frozen before the treatment to give her the chance of having children with her current partner, at a later date.

How are embryos frozen?

Suitable embryos are placed in labelled straws in cooling solutions and then stored in tanks of liquid nitrogen at -196°C. To be suitable to freeze, embryos must:

  • be at the correct developmental stage for the number of days the embryos have been cultured; for example two days after fertilization an embryo should be four cells
  • contain cells which are approximately the same size which indicates regular division
  • contain less than 25% fragmentation
  • have well-defined membranes indicating that they are not undergoing a delicate cellular division at that precise moment

Embryos may be frozen at various stages of development:

  • 2PN, the day after egg collection
  • cleavage, four to eight cells, two or three days after egg collection
  • blastocyst, when the multi-cellular embryo has hatched, usually five or six days after egg collection

Embryos are stored in cryocans at CARE IVF which are regularly inspected

For how long may embryos be stored?

Legislation determines the time that CARE IVF, with patients’ consent, is allowed to store embryos. Currently embryos may be stored initially for 10 years. Under exceptional circumstances, storage may be extended up to 55 years from the date of freezing (to be reviewed every 10 years), if the patient, partner or someone to whom the embryos are allocated is prematurely infertile or likely to become prematurely infertile.