• With a frozen embryo transfer, there’s no need to go through an entire IVF cycle again
  • The embryo transfer is a quick and easy procedure which does not need sedation and feels similar to a smear test
  • Consensus is that in most cases, FET success rates are at least as high as fresh embryo transfer success rates

A frozen embryo transfer (FET) is a cycle in which the frozen embryos from a previous fresh IVF or donor egg cycle are thawed and then transferred back into the woman’s uterus.

If you are undergoing an In Vitro Fertilisation (IVF) or ICSI cycle, you may produce extra embryos that are unused in your current cycle. Frozen embryo Transfer (FET) is a relatively simple procedure which allows you to store any good quality unused embryos, so that you may use them in a future cycle.

With a frozen embryo transfer, there’s no need to go through an entire IVF cycle again. You already have embryos, so there’s no need for your ovaries to be stimulated and eggs recovered as before. 

There are two possible options for performing a frozen embryo transfer: a natural-cycle and a medicated one. Natural cycle is available to women with regular ovulation and monthly menstrual cycles, so patients with predictable regular cycles, frozen–thawed embryo transfer is commonly performed during a natural cycle. Once the precise date of ovulation is set, then the uterine lining should be receptive to embryo transfer 5 days later. In this way, the embryos can be replaced at approximately the time when they would normally implanting.

The problem with natural-cycle is that the optimal time for implantation may fall at an unpredictable time during the laboratory work schedule. In addition it demands frequent patient monitoring around the time of ovulation. Frozen embryo replacement in a natural cycle involves monitoring the natural ovulation using ultrasound scans. After ovulation takes place, the frozen embryos are thawed and transferred back into the womb at the right time.  

For women who have an irregular cycle, a medicated cycle using hormone replacement therapy is often recommended as it can prepare the uterus lining for embryo transfer. 

The Frozen Embryo Transfer procedure

The embryo transfer is a quick and easy procedure which does not need sedation and feels similar to a smear test. 

The number of embryos to be transferred depends on a number of factors such as age, previous failed attempts and the quality of embryos . The standard period that you can store your embryos for is ten years. Please note that not all embryos will survive the freezing and thawing process. Although most embryos do survive the freeze thaw process, some may not survive. Success rates are comparable to fresh embryos. Pregnancy rates have not been shown to differ significantly between both methods.

Frozen embryo replacement costs significantly less than a normal IVF/ICSI cycle as no egg collection or fertilisation is necessary. The advantages of natural cycle are its simplicity with no injections and minimal additional medications with an overall cost savings compared with a medicated one. Recent studies have provided a great reason for hope when it comes to frozen embryo transfers. Consensus is that in most cases, FET success rates are at least as high as fresh embryo transfer success rates. There is no known evidence to suggest an increased risk to babies born from frozen embryos. 

Various decisions about your embryos, including how many to transfer to your womb and when to transfer them should be made. Selecting only the best quality embryo(s) or whether to put 1 or 2 embryos back in the womb, but also at what stage of development to transfer your embryo(s) back to the womb. 

If your periods are irregular, then drugs to suppress your natural cycle and trigger a ‘false’ period may be used. You are then given medication to help prepare the womb lining which when it becomes ready, your embryo(s) will be thawed and transferred to your womb.

Blastocyst transfers have a higher pregnancy rate. It’s best practice to have only one embryo put back. Single embryo transfers do not reduce success rates. The best quality embryo to the womb will be transferred since embryos vary in quality, those that are of the best quality are more likely to implant in the womb and lead to a pregnancy. If more than one embryo is available, the embryologist will use their expertise to select the best quality embryos using criteria such as the number of cells present, how fast the cells are dividing, whether the cell division is even and whether there are any fragments of cells present. 

If you have more than one good quality embryo available, it’s now best practice for most women to have only one embryo put back in the womb and freeze the others (called an elective single embryo transfer or eSET). This is to reduce the chance of you having a multiple birth, which can pose serious risks to the health of both mum and babies. In some cases, the doctor may decide it’s appropriate for women to have more than one embryo put back. This is typically for older women who are less likely to be successful overall and are therefore less likely to have two embryos successfully implant in the womb. 

Embryos can also be frozen to preserve  fertility so it may be possible to have a baby at a later date. The standard storage period for embryos is normally 10 years. 

Not all embryos are suitable for freezing so only good quality embryos will be chosen to freeze. 

The embryos are put in a special solution containing substances (cryoprotectants), which help to draw water out from the embryo and provide protection in the cells. This protects them from damage caused by ice crystals forming. They’re then frozen using fast freezing (vitrification) and stored in tanks of liquid nitrogen until you’re ready to use them.

Freezing embryos using cryopreservation used to be a somewhat risky procedure, where the chances of an embryo surviving both the freezing and thawing process was unpredictable. This was because the freezing method used was comparatively slow and the embryo’s delicate cells could be easily damaged by developing ice crystals.

About a decade ago, a new, advanced freezing method was developed, called vitrification, which is a “flash freezing” process where the embryos are instantly cooled to freezing in seconds. Special advanced media is also used to help protect the cells, using substances known as cryoprotectants, which discourage ice crystals and allow the embryo’s cells to freeze smoothly. With these developments, frozen embryos now have much higher survival rates when thawed (over 95%), and much higher pregnancy rates, too.

Dr Rami Wakim has worked as a fertility specialist for over 25 years and has worked with some of the most esteemed clinics in the world. For more information, visit www.ramiwakim.com

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