Frozen Embryo Transfer (FET)



Frozen embryo transfer is the most advanced technique for cryopreservation nowadays. Extra embryos that are not used during initial IVF procedures can be frozen and then transferred to your uterus later.


Frozen embryo transfer (FET) is now widely available throughout the world. Frozen embryos would be stored in liquid nitrogen tanks at -196℃. Patients fail in artificial cycle of FET may consider using natural cycle for next time.


Procedure for freezing embryos


After a successful IVF cycle, which may have included Intracytoplasmic sperm injection (ICSI), one or two of the best embryos or blastocysts that developed are transferred into the uterus. If you still have several high-quality embryos left, they can be frozen and stored for future use.


Embryos are frozen using vitrification technique. This involves rapid freezing to prevent any ice crystals from forming in the cells of the embryo. The faster the freezing process, the more likely the frozen embryo transfer will result in a successful pregnancy.


Frozen embryos are stored in a special solution in a sterile vial inside a container of liquid nitrogen, which is at -196℃. Labelling is done very carefully and the embryos can only be thawed with a written consent from both parents. Embryos are known to survive for many years once they are frozen, and embryos are currently allowed to be kept for 10 years.


Who would benefit from FET?


  • Extra embryos available after embryo transfer
  • Uterine condition is not suitable for embryo transfer in this cycle, such as Ovarian hyper stimulation syndrome (OHSS) or substandard endometrium development. The gap between OPU and frozen embryo transfer allows her body to recover before introducing the embryos back into the uterus, resulting in higher chance of pregnancy.
  • Poor physician health conditions which are not suitable for embryo transfer in this cycle, such as fever and diarrhoea
  • If patients’ ovaries no longer function properly due to various conditions, such as radiotherapy, chemotherapy or ovariectomy. They may chose to frozen the embryo to preserve their fertility.


Natural Cycle


When you are undergoing an IVF cycle and has been advised to use your fresh embryos, that means your uterus is ready for embryos transfer.


If you are advised to go for a frozen embryo transfer cycle, you may have to be careful about the timing of the transfer and your natural menstrual cycle. This could be difficult because it requires constant monitoring to find out when you ovulate so that frozen embryo transfer can be done on a right day.


Artificial Cycle


Otherwise you could get your uterus ready by using hormone treatments which involves:


  • Suppressing your own natural hormone production using a drug that acts on your pituitary gland. This is usually done for two weeks so that you do not ovulate unexpectedly before your frozen embryo transfer.
  • Having oestrogen treatment in the form of tablets, injections, skin patches or vaginal suppositories to mimic a normal menstrual cycle by encouraging the lining of the uterus to thicken. The form of oestrogen given usually depends on your infertility clinic; all work equally well.
  • Monitoring your uterus using trans-vaginal ultrasound and having regular blood tests to check your blood levels of oestrogen.
  • Progesterone treatment, which starts once the lining of your uterus has thickened. Taking the drug to suppress your pituitary gland usually stops at this point, but you carry on taking oestrogen and progesterone together to fully mature the lining. Progesterone can be given in the form of a vaginal gel or suppositories or Tablets tend not to be used because the form is not as effective as the other three.




Frozen embryo transfer then takes place several days after progesterone treatment begins. The uterus will only be ‘ready’ for a short time, and if this window of opportunity is missed, the treatment cycle needs to start again.


Timing varies depending on whether the frozen embryo transfer is being done with 2–3 day embryos or 5–6 day embryos (frozen blastocyst embryo transfer). Blastocysts are usually transferred on day 5 of progesterone treatment.