What is a Frozen Embryo Transfer (FET)?
FET is a cycle in which frozen embryos from a previous fresh IVF cycle are thawed and then transferred back into the woman's uterus. Overall, 40% of all fresh IVF cycles completed at AFCT result in high-quality blastocyst-stage embryos on day 5 or 6 that are available for freezing. The chance of having embryos available to freeze greatly depends on age. For example, in 2011, 50% of cycles in which the woman was 38 years old or younger had embryos available for freezing, while only 12% of women over the age of 40 had blastocyst-stage embryos available for freezing.
In recent years, the success with FET has increased substantially, making it a viable option to consider before moving to another fresh IVF cycle. Over the past few years, our team has received several questions from patients about FET and how they work.
What are the benefits of Frozen Embryo Transfer?
·There are now several studies that confirm the theory that we can obtain significantly higher pregnancy rates when we wait and transfer the embryos in a frozen cycle (about 6 weeks from egg retrieval) as compared to transferring the embryos in a fresh cycle. This is particularly important in patients who we anticipate will have a good response to ovarian stimulation (estrogen levels greater than 5000 pg/mL).
·When we stimulate the growth of the oocytes (eggs), we are increasing the estrogen levels to 10-20 times higher than normal in a natural cycle. Such high estrogen levels appear to have a negative impact on the uterus, which now may not be aligned with the growth of the eggs. High estrogen levels inhibit the effects of progesterone, which is the most essential hormone in the process of implantation and pregnancy.
·Patients who have polycystic ovarian syndrome (PCOS), naturally have a higher number of eggs compared to other patients prior to egg stimulation. The higher number of eggs leads to high levels of estrogen in the end of IVF stimulation. Therefore, patients with PCOS greatly benefit from frozen embryo transfer.
·The risk of ovarian hyperstimulation syndrome increases with high estrogen levels, and this is also true in patients with PCOS. In this syndrome, patients experience severe abdominal discomfort, fluid collection in the abdomen, and shortness of breath, which may require hospitalization. When we perform frozen embryo transfer, we can slightly alter the stimulation protocol and almost completely eliminate this syndrome.
·Some data suggests that the pregnancies resulting from frozen embryo transfer are healthier and the infants are about 100 grams bigger.
·The process of freezing and thawing the embryos is called vitrification and has been perfected to the point where almost all the embryos that we freeze can now survive the thawing process.
·The process of frozen embryo transfer involves only the preparation of the uterus so that it can be aligned with the embryos that are frozen. This can be easily accomplished by administering estrogen pills for about 2 weeks. The embryos will be thawed and transferred according to a set calendar that is provided for the patient.
What are the success rates for a Frozen Embryo Transfer?
The success rates of an FET cycle are nearly the same as a fresh IVF cycle and have the same primary indicator for success: the maternal age at the time of embryo freezing. Many patients wait several years between the initial freeze of their embryos and attempting a subsequent FET cycle. Any patient, regardless of the amount of time between embryo freezing and thawing, can expect nearly the same potential for success as they experienced with a fresh IVF cycle, which the frozen embryos initially came from. Patients can expect the same chances of success because frozen embryos are suspended in time and do not age.
Women 37 years and younger can expect about 50% delivery rate per thaw: about half of women give birth after a FET cycle. This rate declines with increasing maternal age at the time of embryo freezing.