Frozen embryo transfer (FET) is a procedure in assisted reproductive technology (ART) where a previously frozen embryo is thawed and transferred into the uterus. This method uses embryos from a prior in vitro fertilization (IVF) cycle. The first successful pregnancy using a cryopreserved embryo occurred in 1983, and advancements like vitrification have improved embryo survival rates.
The Process of Frozen Embryo Transfer
The FET process begins by preparing the uterus to create an optimal environment for embryo implantation. This involves hormonal medications, typically estrogen and progesterone, to thicken and mature the uterine lining. Estrogen is usually started early in the menstrual cycle to promote endometrial growth, aiming for a thickness of at least 7-8 mm.
Following endometrial preparation, progesterone administration begins, which further matures the uterine lining and opens the “window of implantation.” This hormone is crucial for creating a receptive environment for the embryo. Ultrasounds and blood tests monitor uterine lining thickness and hormone levels to ensure readiness for transfer.
The frozen embryo is thawed in the embryology laboratory on the day of or the day before the transfer. Modern vitrification techniques lead to high survival rates, typically 90-95% of embryos. Once thawed, the embryo is assessed for viability and ready for transfer.
The embryo transfer is a generally painless, outpatient procedure that does not require anesthesia. A thin, flexible catheter containing the thawed embryo(s) is guided through the cervix into the uterine cavity, often with ultrasound guidance for precise placement. After the transfer, patients are advised to rest for a few days and continue prescribed medications to support implantation.
Key Advantages of Frozen Embryo Transfer
Frozen embryo transfer offers several benefits compared to fresh embryo transfers, primarily due to an optimized uterine environment. By separating ovarian stimulation from embryo transfer, FET allows hormone levels to return to a more natural state, which can improve uterine receptivity. This may also lead to higher implantation rates.
A significant advantage of FET is the reduced risk of Ovarian Hyperstimulation Syndrome (OHSS). In fresh cycles, ovarian stimulation can lead to OHSS, a serious complication. Freezing all embryos and performing the transfer in a subsequent cycle helps patients avoid this risk.
FET also provides an opportunity for preimplantation genetic testing (PGT). Embryos can be biopsied and tested for genetic abnormalities before freezing, allowing selection of genetically normal embryos for transfer. This process requires embryos to be frozen while awaiting test results, making FET a necessary component for PGT.
Furthermore, FET offers flexibility in family planning. If surplus embryos are created during an initial IVF cycle, they can be cryopreserved for future attempts. This eliminates the need for another full ovarian stimulation and egg retrieval procedure, allowing individuals or couples to space out pregnancies or attempt conception again without undergoing the entire IVF process from the beginning.
Understanding Success Rates and Outcomes
The success rates of frozen embryo transfer cycles vary, influenced by several factors. Patient age at the time of embryo freezing is a significant predictor, with younger individuals generally having higher success rates. For instance, clinical pregnancy rates can be around 50% for women under 35, decreasing for older age groups.
Embryo quality is another crucial determinant of FET success. High-quality blastocysts (embryos cultured for 5-6 days) generally have higher implantation and pregnancy rates compared to cleavage-stage embryos (day 3 embryos). The number of embryos transferred also plays a role, with double embryo transfers sometimes showing higher clinical pregnancy rates than single embryo transfers, though this must be balanced against the risks of multiple pregnancies.
Uterine health, particularly the endometrial thickness on the day of transfer, is also important. An endometrial thickness of at least 8 mm is often considered favorable for implantation. While success rates vary by clinic and individual circumstances, overall clinical pregnancy rates in FET cycles can range from approximately 34% to 46% or higher.
After the embryo transfer, there is a waiting period, typically 8-10 days, before a pregnancy test measures the human chorionic gonadotropin (hCG) hormone. If positive, further blood tests and ultrasounds confirm a viable pregnancy.