A frozen embryo transfer (FET) cycle involves the thawing of one or more embryos (frozen during a previous fresh treatment cycle) and the transfer of those embryos back to the uterus in order to try to establish a pregnancy.

Freezing and thawing of embryos and FET are considered routine procedures at Life Fertility Clinic.

In a typical IVF or ICSI cycle, stimulation of the ovaries to produce many eggs often leads to the production of more embryos than can be safely transferred to the woman’s uterus at one time. It is routine practice therefore to select the best one or two embryos for transfer and to freeze any suitable surplus good quality embryos. These can then be thawed and transferred at a later date in the event that the fresh cycle is unsuccessful or perhaps siblings are sought.

In some circumstances, it may have been necessary to postpone a fresh embryo transfer completely and freeze all suitable embryos rather than transfer them to the uterus. This may have been considered necessary during the fresh cycle if the lining of the uterus (endometrium) had not developed optimally or where the woman was at risk of developing ovarian hyper-stimulation syndrome (OHSS).


Once you are ready to begin your FET cycle you should contact your nurse coordinator. They will review your fertility specialist’s instructions for that cycle prior to you commencing and will also assist you in making sure you have completed all of the appropriate paperwork for the cycle (e.g. consent and Medicare forms).

It is important to note that none of the procedures relating to your FET cycle can proceed unless you and your partner have signed and returned the appropriate consent forms.

During a FET cycle frozen-thawed embryos are transferred into the uterus after the preparation of the endometrium using either a ‘natural cycle’ or a ‘hormone replacement therapy cycle’.

– Natural Cycle

In a natural FET cycle, development of the endometrium is allowed to occur naturally as part of the woman’s normal cycle. The developing follicle and the thickness of the endometrium are monitored during this cycle by ultrasound, and once they have reached an appropriate size/thickness, an injection is usually given to initiate ovulation. The embryo thawing and transfer will then take place in the week or so following this; the exact timing of which will depend on a patients individual treatment plan and the stage of development at which their embryos were frozen. Progesterone medication, in the form of either an injection or pessaries, is often given during the second half of the cycle to help support the endometrium.

– Hormone Replacement Therapy (HRT) Cycle

During a HRT cycle, appropriate development of the endometrium is achieved using a combination of oestrogen and progesterone medications. Oestrogen medications, often given in the form of a tablet, aid the development of the endometrium during the first part of the cycle and are continued throughout the cycle. Progesterone medications are used during the second part of the cycle to help support the endometrium. The development of the endometrium is monitored by ultrasound to determine the appropriate time to introduce progesterone medications and schedule the embryo thawing and transfer. This typically occurs in the week or so following the commencement of progesterone, but can vary depending on individual circumstances and the stage that your embryos were frozen.


The laboratory will thaw your embryos so that the stage of the embryo development corresponds to your menstrual cycle. Since embryos can be frozen on day one after insemination (pronuclear stage embryo) or at any stage up to and including the blastocyst stage (five or six days after fertilisation) the exact timing of thawing will depend on the stage that your embryos were frozen.

At Life Fertility Clinic embryos are most commonly frozen at the blastocyst stage. Embryos frozen at this stage are usually thawed in the morning on the day of your embryo transfer.


The embryo transfer involves a fine tube (called a catheter) being passed through the cervix. This allows the embryo(s) to be passed through the catheter into the uterus where they are placed in a very small volume of culture medium. This technique usually does not require any sedation and most patients do not experience any more than mild discomfort.


Your nurse coordinator will speak to you following the embryo transfer and review the post embryo transfer instructions from your fertility specialist, including your medications and any lifestyle considerations.

A pregnancy blood test will be ordered by your fertility specialist for around 11 to 14 days after your embryo transfer to determine if you are pregnant. The blood test measures the hormone human Chorionic Gonadotropin (hCG), that is produced during pregnancy. It is important that you wait until the advised date to have your test. If you were prescribed injections to trigger ovulation earlier in your FET cycle these can lead to a false positive result if testing is performed to soon. Equally a false negative result may occur if the test is performed before hCG levels in pregnancy are high enough to be detected.

Regardless of the type of FET cycle you are having, it is very important that you continue the medications as prescribed by your fertility specialist until you have received the results of your pregnancy test.

Some patients experience bleeding or spotting before their pregnancy test. This is not necessarily a period and, although there are many possible causes, it can sometimes be the result of the developing embryo implanting in the endometrium. It is very important therefore that you continue your medications as prescribed until advised otherwise by your nurse co-ordinator or fertility specialist.


The main risk associated with freezing and thawing of embryos is cell damage to the embryo. If a large number of cells are damaged during the freeze-thaw process the embryo may not survive.

Embryo survival rates are generally highest (approx. 90%) for blastocysts that have been frozen by a process called vitrification. Embryos frozen either at the pronuclear stage, or at the 2 to 8 cell stage are usually frozen by a slightly different technique called ‘slow freezing’ and approx 80% of these embryos survive the thawing process. The majority of frozen embryos at Life Fertility Clinic are frozen at the blastocyst stage using the vitrification technique.

Embryo survival does however vary between patients and, irrespective of the technique used or the stage at which they were frozen, it is possible that none of a couple’s embryos will survive the freezing and thawing process.

To date, there is no evidence to suggest that the freezing and thawing of embryos has resulted in an increased risk of miscarriage or congenital abnormality over that expected for natural pregnancies.


Success rates for frozen-thawed embryo transfer cycles vary. However, the implantation rate for thawed vitrified blastocysts is approximately the same as for fresh blastocysts.



For more information or queries regarding any of the services offered at Life Fertility Clinic, please contact us.