What is Frozen Embryo Transfer?
A frozen embryo transfer (FET) is when cryopreserved embryos are thawed and transferred to a woman’s uterus. Patients may ask to freeze and store any extra embryos that are not used in their first cycle for a variety of reasons. They may want a second child or have had a failed initial cycle. In some cases, their hormonal profile and/or their uterine lining is not optimal during a fresh IVF cycle. Before the embryos are transferred, blood tests and ultrasound scans are conducted. This enables the physician to identify the appropriate point in the patient’s cycle to transfer the embryos. Some patients may need to take hormone medication to prepare for the transfer.
FET with Hormonal Support
For patients with irregular cycles and/or have time constraints, their physician may recommend FET with hormonal support. In this case, hormones such as estrogen and progesterone are administered to suppress or control ovulation as well as ensure that the patient’s uterine lining is prepared for the transfer. Embryo transfer takes place on the third or fifth day of progesterone support.
FET Natural Cycle
FET natural cycles, on the other hand, do not involve medications that control ovulation, instead it is based on when ovulation naturally occurs. The patient’s doctor closely monitors her menstrual cycle through blood tests and ultrasound to identify the optimal day for embryo transfer. The frozen embryos are thawed and are transferred to the uterus.
Until the flash freezing process, known as vitrification was introduced a decade ago, frozen embryos had a very low implantation and live birth rate. There were various reasons for the low numbers. One of the primary issues was the freezing process itself. The slow freezing method enabled ice crystals to form within the embryo, which damaged it to the point at which they were not longer viable. Fortunately with the development vitrification, over 90% of the frozen embryos survive thawing and are the same quality as fresh embryos. Vitrification is the process of freezing eggs and embryos with extremely rapid cooling, which is too fast for crystals to form. The improvements in the freezing process and in the selection of embryos process, frozen embryos are now frequently used and proven to result in successful pregnancies.