A Frozen Embryo Transfer (FET) is a cycle in which frozen embryos from a previous IVF treatment are thawed and then transferred into uterus.
To prevent high-order multiple pregnancies
In a IVF cycle, generally 10-15 eggs are obtained by controlled hyperstimulation and result in a number of high-quality embryos available for transfer. Transferring multiple embryos increases the risk of a high-order multiple pregnancy (like triplets or quadruplets). For this reason one or sometimes two embryos are transferred and all the remaining good quality embryos are freezed to increase the chance of future pregnancy.
High Risk of OHSS
Ovaries are sometimes overstimulated during controlled stimulation therapy by fertility drugs in some patients especially in polycystic ovarian syndrome patients (PCOS). This is called ovarian hyperstimulation syndrome (OHSS) and the fresh embryo transfer is cancelled and all embryos are cryopreserved because pregnancy can exacerbate OHSS. Once the patient has recovered from OHSS, a frozen embryo transfer cycle can be scheduled.
Medical necessity due to anticipated poor endometrial receptivity
During controlled ovarian stimulation in an IVF cycle, in some cases, the woman has elevated progesterone levels or has a lining under 7 mm at the time of trigger. In these situations it is advised to freeze all embryos followed by a FET. Thus, the uterus/endometrium will be prepared with drugs in a way that it will be more recipient and ready for implantation in a later frozen embryo transfer cycle.
A Second Embryo Transfer
If the fresh embryo transfer fails then the couple have a chance for a second thawed embryo transfer.
If the couple want to continue expanding their family after giving birth cryopreserved embryos can remain in the laboratory indefinitely.
Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are the techniques that are used in IVF to screen embryos for specific genetic diseases or chromosomal defects. On day three or five post egg retrieval a biopsy is done from the embryos. Sometimes, the results get back in time to do a fresh embryo transfer. However, if a day five biopsy or the genetic testing is complex and requires more time, all embryos that were biopsied will be cryopreserved. After the results are obtained which embryos are to be transferred in FET-IVF cycles are decided.
Fresh Embryo Transfer Cancelled
Fresh embryo transfer might also be canceled for some various medical reasons. The woman who is a candidate for motherhood might not be able to have fresh embryo transfer because of some medical illness such as flu or another illness after egg retrieval but before transfer.
How is endometrium prepared for implantation?
Endometrium is prepared under natural cycle or by estradiol and progesterone hormones either by mouth or intravaginally, or by patches (FET-HRT).
FET Natural Cycle
In a FET natural cycle embryo transfer is scheduled based on when ovulation naturally occurs without using medications to suppress or control ovulation. The cycle is closely monitored at the fertility clinic with ultrasound and blood tests. When ovulation is detected, progesterone supplementation is started, and the transfer date is scheduled. Thawed embryo transfer timing is planned a number of days after ovulation depending whether the embryo was frozen on day 3, 4 or day 5 post-egg retrieval. Although the advantage is the absence of estrogen supplementation, this protocol entails more frequent visits to the clinic, less cycle control and flexibility and holds a higher risk of cycle cancellation up to 6%.
FET With Hormonal Support
With the begining of the menstrual bleeding estrogen supplementation is started. The purpose is to prepare a healthy endometrial lining. Estrogens may be administered orally, vaginally and parentally (transdermal route) and both natural as well as synthetic estrogens may be used. Estrogen supplementation is continued for about two weeks, and once the proliferation of the endometrium with the administration of estrogens is considered sufficient as endometrial thickness measured by transvaginal ultrasound, progesterone is initiated to promote the final phase of endometrial preparation prior to embryo transfer. This can occur via progesterone in oil injections or possibly with vaginal suppositories. The monitoring during a FET-IVF cycle is significantly less than it is during a classic IVF cycle.
What cycle day is frozen embryo transfer?
The embryo transfer is scheduled based on when progesterone supplementation was started and on what stage the embryo was cryopreserved. For example, if the embryo was cryopreserved on day 5 post-egg-retrieval, then the frozen embryo transfer will be timed for day 6 after progesterone supplementation starts.
How long does it take to prepare for a frozen embryo transfer?
A frozen embryo transfer by itself requires about three weeks.
What is the process of a frozen embryo transfer?
The Frozen Embryo Transfer is a simple and painless procedure. It involves thawing and then transferring the embryo into uterus through a tiny and smooth catheter for hopeful pregnancy.
As there will be no egg retrieval ovarian stimulation medications are not used. In FET cyles estrogen and progesterone is started during the first days of the period to thicken the lining of the uterus to allow implantation after embryo transfer. There will be no egg retrieval requiring anesthetic drugs.
FET cycles are often less stressful than fresh cycles as:
FET cycles are less costly than fresh cycles as no ovarian stimulation medications are used and no egg retrieval procedure is performed.
Provide Additional Cycle Opportunities
If the first frozen transfer is unsuccessful, the rest frozen embryos may provide the possibility of trying again a frozen embryo transfer cycle without ovarian stimulation medications and another egg retrieval procedure. As a result patients can achieve live birth from a single stimulated cycle.
Frozen embryo transfer is usually done on either the 4th or 5th day of progesterone, depending on which day following fertilization the embryo had been frozen.
Implantation of a frozen thawed embryo is more successful than embryo transfer in a fresh cycle because the environment in the uterus is more 'normal and natural' than an IVF cycle where hormone levels are often very high.
The only risk is the risk of multiple pregnancy, which can cause serious health problems for the mother and babies. This risk can be reduced by transferring only one embryo to the uterus, a process known as elective single embryo transfer or eSET. Therefore FET is just as safe as fresh embryo transfer cyles.
Unfortunately, some embryos may not be healthy after freezing and thawing process, or very rarely, no embryos will survive. All embryos might not have the same potential to form a pregnancy, and each embryo is affected differently by the freezing and thawing processes. Top quality embryos are more viable than embryos of a poorer quality. The transfer of a frozen–thawed top quality embryo lead to a similar live birth rates as in the fresh cycles.
Frozen embryos are stored in liquid nitrogen tanks at a temperature of -196 ° C. This means they do not deteriorate over time during storage as long as they are kept stable in the case.