The Patient's Guide to Embryology

Frozen embryo transfer

The success rates of frozen embryo transfers have increased dramatically and the chance of getting pregnant is now equal to a fresh embryo transfer, if not better.

Frozen embryo transfer

A frozen embryo transfer (also known as FET or FER) is the same procedure as a fresh embryo transfer but a frozen embryo is thawed out and placed into the uterus instead of a fresh embryo. These can be surplus embryos which were not used in a fresh cycle or embryos that were intentionally frozen for later use such as for PGT-A testing or in an elective freeze-all cycle.

Frozen embryo transfer cycles are quicker and more straightforward than an IVF cycle with fewer drugs and less monitoring which is why it is so beneficial to have additional embryos frozen from your fresh cycle. It gives you extra attempts to get pregnant without going through the whole IVF process again. Embryos can stay frozen for decades without changing quality and whatever age you freeze the embryos at locks in the success rates for that age – ie. if you have a frozen embryo transfer at 42 years old with an embryo frozen when you were 35, you have the same chance of success as you did at 35. This is a huge advantage in IVF since female age is one of the most important predictors of success.

Key points about frozen embryo transfers:

  • Can be scheduled to suit you
  • The procedure is the same as a fresh embryo transfer
  • Around 95% of embryos survive freezing and thawing
  • Fewer/no injections
  • Many clinics report higher success rates than fresh embryo transfers

The procedure

It is possible to have a frozen embryo transfer as part of a natural cycle where you don’t need to take any medications, a modified natural cycle, or as part of a hormone-regulated cycle:

  • Natural cycle FET: The lining of the uterus is allowed to naturally thicken and the embryo will be thawed and transferred as if it were a natural pregnancy based on when you ovulate. You will have ultrasound scans to monitor the uterus and ovaries and then you will need to test for ovulation at home. The date of the embryo transfer is planned for 5 days after ovulation (if you are having a blastocyst transfer) to mimic the timings of a natural pregnancy. While this is a cheaper and more straightforward protocol, ovulation is unpredictable so it is difficult to get the timings exact, particularly if your periods are irregular and this can affect planning and outcomes.
  • Modified natural cycle FET: The same process is followed as in a natural cycle FET but ovulation is triggered artificially. When your ultrasound scans show the lining of the uterus is the correct thickness and the biggest follicle on the ovary is the correct size, a trigger injection is given to cause ovulation and the embryo transfer is planned for 5 days later.
  • Hormone-regulated FET: Hormone drugs are used to prepare the lining of the uterus to receive an embryo. There are different protocols depending on your clinic and your personal history but a cycle can involve combinations of estrogen tablets, injections and progesterone pessaries. Typically, estrogen is given on day 1 of your period which will start to thicken the lining of the uterus. Ultrasound scans are used to measure the thickness of the lining and when it is the correct thickness, the embryo transfer can be planned. You will start taking progesterone (commonly a vaginal pessary) in the days leading up to the embryo transfer to mimic the hormone changes in a natural cycle.

There is no difference in the embryo transfer procedure whether using a fresh or frozen embryo. It is a straightforward procedure which feels similar to a smear test and takes around 20 minutes. A speculum is used to find the entrance of the womb and a fine catheter containing the embryo is passed through the cervix. An ultrasound scan is usually done at the same time to ensure the embryo is being placed in the perfect position. The embryo is pushed out of the catheter into the uterus in a tiny volume of liquid and hopefully it will implant into the lining in the next few days.

Thawing the embryo

The technical term for embryo thawing is ‘warming’ and it is not as simple as just removing it from the freezer. Each cell in the embryo needs to be slowly rehydrated as the embryo gradually recovers. The process takes around 20 minutes and it is usually done on the same day as the embryo transfer. Frozen embryos often lose the fluid from their central cavity making them appear crumpled or collapsed. Some embryos re-expand in time for the embryo transfer and others re-expand in the body, but this doesn’t affect their chance of implanting.

If you have more than one embryo cryopreserved, they will likely be thawed in order of their quality with the highest grade embryos being chosen first. Embryos are very resilient and only 1-2% fail to survive after they are thawed.

Fresh or frozen – which is more successful?

In the early days of IVF there was no doubt that fresh embryo transfers were the most successful, but this is now no longer the case. The success rates of frozen embryo transfers have increased dramatically and the chance of getting pregnant is now equal to a fresh embryo transfer, if not better.

The increase in FET success rates is mostly due to a new method of embryo freezing called vitrification. Embryos frozen using the old method had around 80% chance of survival whereas embryos frozen using vitrification have around 95-99% chance of surviving and can stay frozen for decades without changing quality.

Another major advantage of frozen embryo transfers over fresh embryo transfers is that the uterus is thought to be more receptive to implantation. A fresh embryo transfer is typically done 5 days after egg collection when the body is still recovering and the ovarian stimulation drugs are still circulating in the blood, whereas freezing the embryo and delaying the embryo transfer gives the body chance to settle back down so the uterus environment is at its most receptive and better synchronised with the embryo which can improve the chance of implantation. For these reasons, many clinics and people now opt to freeze their embryos and use only frozen embryo transfers, known as ‘elective freeze-all’.

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