Frozen embryo transfer, or FET, is an assisted reproductive technology procedure in which a previously frozen embryo is thawed and transferred it into an appropriately prepared uterus in order to have a baby.
Frozen embryo is a term for embryos that are not transferred during an IVF cycle and are deemed suitable for cryopreservation, or freezing. The best embryos are most commonly selected for transfer in fresh IVF cycles. Should suitable quality embryos remain, they can be frozen for later use. Not all cycles have embryos that are suitable for freezing. The rate of survival following thawing of frozen embryos is approximately 70%, so only good quality embryos should be frozen.
Occasionally a freeze-all cycle may be recommended. Instead of transferring an embryo following IVF, all embryos are frozen for later thawing so that the best surviving embryo can be transferred at a later time. Freeze-all cycles may be suggested for a variety of reasons such as if the woman is ill or has uterine bleeding that could result in the transfer being less likely to achieve a successful pregnancy. Other reasons to do a freeze-all are in cases where either a uterine polyp may jeopardize implantation due to its size and location, or in cases where intrauterine fluid has accumulated, lowering the probability of the embryo being able to adhere to the uterine wall. In cases of ovarian hyper-stimulation syndrome (OHSS), a freeze-all is often recommended as the risk of OHSS severity is significantly reduced if pregnancy does not occur. Still another indication for a freeze-all is in cases where pregnancy has not resulted from multiple prior fresh embryo transfers due to potential detrimental effects resulting from hormonal fluctuations or imbalances.
How is an FET cycle done?
Before starting an FET cycle, the physician needs to assess the uterine cavity using one of three tests:
- Hysterosalpingogram (HSG) – In which x-ray dye is injected into the uterus and the cavity is viewed with x-rays;
- Sonohysterogram (SonoHSG) – In which saline is injected into the uterus and the cavity is viewed with ultrasound;
- Hysteroscopy – In which a fiberoptic scope is introduced into the uterus and the cavity viewed directly.
If any abnormalities are detected, they should be corrected surgically before going forward with a frozen embryo transfer.
There are two protocols used for FET cycles. Both of these protocols use hormones to prepare the uterus for embryo implantation.
The first FET protocol involves pituitary gland suppression using subcutaneous injections of Lupron for approximately two weeks to decrease the chance of ovulation occurring unexpectedly. Estrogen medications are also given to increase uterine lining thickness, preparing it for implantation. When the uterine lining has achieved a targeted thickness, the Lupron is discontinued and progesterone is given. Embryo transfer is usually done on either the 4th or 6th day of progesterone, depending on which day following fertilization the embryo had been frozen.
The second FET protocol does not include Lupron but simply involves the use of estrogen for up to 3 weeks. When the endometrium reaches a targeted thickness and ovulation has not occurred, progesterone is begun, followed by embryo transfer on either the 4th or 6th day of progesterone as in the first protocol.
There is no data that any one protocol works better than another and the protocol chosen is based upon your specific needs.
Frequently Asked Questions
Embryos are frozen at different times following fertilization, depending upon your individual needs. This usually occurs the day of fertilization (2 Pronuclear, 2PN stage), Day 3 following fertilization (5 to 8 cell stage), or Day 5 (Blastocyst Stage of Development). Freezing and thawing are stressful for embryos, and only good quality embryos are selected for freezing. The chance of frozen embryos surviving thawing is 70% to 80%. The embryo selected for freezing is placed in a small straw that is then placed into a cooling chamber of a controlled rate freezer. The embryo is cooled slowly to maximize water extraction from the embryo and prevent ice formation that can damage the embryo. Once frozen, the embryos are stored in liquid nitrogen.
Embryo freezing has been performed since the 1980’s. It is not known how long embryos can be safely frozen, although there have been successful pregnancies even after being frozen for 10 years. Most likely they can be stored indefinitely.
A careful inventory is kept for all embryos in storage by our laboratory. The charge for embryo freezing includes the cost of storage for one year. Patients are then billed annually. It is the patient’s responsibility to notify us of any change in address.
Patents no longer desiring to store their frozen embryos have several options. Ideally, they can be used in a frozen embryo transfer cycle. Should you no longer desire future children, they can be donated to another couple seeking a baby. While we do not do embryo donation ourselves, we will be happy to send them as directed by you to agencies that specialize in matching frozen embryo donors and recipients. Frozen embryos may also be discarded as medical waste. Alternatively, frozen embryos can be donated for scientific research, in which case they are thawed, studied and then discarded. Patients also have the option of donating their frozen embryos for embryonic stem cell research.
The disposition of embryos require your direction and written consent signed by both partners, as applicable, notarized, or witnessed by one of our staff. Our embryologists initial and date the consent form and other storage documents, attesting that they performed and witnessed the disposition according to the patients’ wishes. The paperwork is kept in the laboratory files. If requested on the consent form, written confirmation of the disposal of the embryos is sent to patients.
Recent evidence from Denmark (Fertil Steril. 2010;94:1320-1327) looking at infant outcome of 957 pregnancies born after frozen embryo transfer concluded that neonatal outcome was better in terms of higher birth weights and longer gestational ages for babies born following an FET than occurred for babies born after a fresh embryo transfer. This indicates that FET cycles are at least as safe and may be better than fresh cycles.