Frozen Embryo Transfer

Spare embryos from your IVF treatment can be frozen, ready to be thawed and used in future cycles, depending on their quality. Patients who want to use their previously stored embryos go through what’s called a frozen cycle. There are two types of frozen cycle; natural or stimulated (FET-HRT).

Natural cycles

In a natural frozen cycle there are no stimulation drugs to take before embryo transfer, but you do have a one-off injection after transfer, to aid luteal support. We monitor your menstrual cycle by taking blood tests from around day 10 of your period, to ascertain when your natural ovulation happens. With this information, we can replace the embryo(s) at the point in time when implantation is most likely.

FET-HRT cycles

With a stimulated FET-HRT cycle, you take medication to prevent you from ovulating. All your treatment arrangements will be made before your period starts. One Day 1 of your bleeding you call the clinic, and confirm that you’ll start taking medication to prepare your womb for the embryo transfer. You’ll have a scan on day 8-10 of your period to check the thickness of your endometrium. Once it is the right thickness we can introduce further medication, start the embryo thawing process and book your transfer.

Embryo thawing and transfer

Embryos are thawed in the laboratory and assessed as to whether they have fully survived the thawing process. Our embryology team will inform you of how well your embryos have survived and whether they are of a good enough quality for your transfer to go ahead. Sometimes embryos do not survive the thawing process. If this is the case, you will be offered a follow-up appointment with one of our consultants to discuss the results and further options.

Hopefully the embryos will then be ready to transfer the same as in a fresh cycle. Our frozen cycle success rates have been improving year on year and are now almost as high as our fresh cycle rates.

Learn more about FET and other types of cycle in this patient journey.

What proportion of embryos survive the freezing/thawing process?

We find approximately 80% of frozen embryos survive the freezing/thawing process. This can vary depending on what stage of development the embryos were at when they were frozen, and the affect freezing may have on different embryos.

Do frozen embryos deteriorate over time in storage?

Frozen embryos are stored in liquid nitrogen at a temperature of -196°C. This means they are held in a suspended state and do not deteriorate with time in storage.

How many embryos should I thaw for my frozen embryo replacement cycle?

Our aim is to give you the best chance possible of pregnancy with each frozen embryo replacement cycle. Not all embryos have the same potential to form a pregnancy, and each one is affected differently by the freeze and thaw process.

If you have embryos frozen at the day 2-3 stage of development we will usually recommend thawing at least 4 and culturing them in our incubator until the blastocyst stage to help us choose the best embryos.

If you have embryos frozen at day 5 of development (blastocysts), we will usually recommend thawing only the number of embryos you wish to have transferred, because the survival rate is high. Blastocysts are thawed and transferred back to the uterus on the same day.