Frozen Embryo Transfers (FETs)- a good option for me?
Following in-vitro fertilization, you may have your embryos transferred via Fresh or Frozen Embryo Transfer (FET). Over the past decade, the number of FETs reported to the Society for Assisted Reproductive Technology (SART) increased dramatically- for example, from 2006-2012, FETs increased 82.5%, compared to a 3.1% increase in fresh transfers. In conjunction, the percentage of live births per FET increased more than fresh transfers*. There is a clear trend to utilize FET across the country, likely from multiple simultaneous causes including:
- Improved cyropreservation (freezing) techniques
- Prevention of OHSS in high-responders
- Increased use of genetic screening
- Decrease in number of embryos used in each transfer in an effort to reduce multiple gestations (therefore more embryos are leftover to freeze)
Why have a frozen embryo transfer?
Implantation is a key step in IVF success. The main factors affecting implantation are embryo quality and endometrial receptivity- and recent evidence suggests that the supra-physiologic (above normal) estradiol levels that occur during IVF cycles may be associated with decreased endometrial receptivity. In IVF clinics, FET is sometimes proposed as a means to transfer an embryo or embryos into a more “physiologic” environment. While this rationale is compelling based on ongoing research, it is not yet proven, and there is not yet definitive evidence to say precisely what threshold of “supra” physiologic estradiol levels should be used to recommend FET. Randomized, controlled-trials are ongoing to investigate this question more thoroughly.
Some reasons you may undergo FET:
- You are planning to have pre-implantation genetic screening or diagnosis in order to choose the best embryo for transfer (the results take time to process).
- Your MD is concerned for your risk of ovarian hyperstimulation syndrome (OHSS).
- There is another hormonal or medical reason for you to undergo FET.
- You and your partner may be deferring childbearing but desire to preserve your fertility.
- Previous failed fresh embryo transfer.
How are embryos frozen?
Embryos at NRM are frozen through the process of vitrification, a modern, more efficient and effective manner to freeze embryos as opposed to the historical method of slow-freezing. Vitrification, or flash-freeze of the embryos in liquid nitrogen, suspends the embryos in a glass-like state to avoid ice-crystal formation associated with freezing. In this manner, damage to the embryo is reduced and the integrity of the embryo is preserved. Vitrified embryos may be stored indefinitely.
What are the pros/ possible benefits?
Depending on the patient and cycle characteristics, potential benefits include:
- Preventing/ eliminating late- onset OHSS
- Allowing time for genetic test results
- Restoring endometrial receptivity thus increasing the implantation and pregnancy rates
- Separating the transfer from the stress of ovarian stimulation
- Recent studies show reduced maternal and fetal risks with FET compared to fresh transfer, such as:
- Decreased bleeding complications during pregnancy
- Decrease in low birth weight infants
- Decrease in preterm labor and delivery
What are the cons/ possible risks?
- In a small percentage of cases, an embryo may not survive the freeze-thaw process.
- Many reports have found that babies born after FET weigh more than babies born after fresh transfer. While this is usually a good thing, some studies show an increase in macrosomia, or infants that are large for gestational age.
- Additional time to pregnancy, inconvenience.
What is the success rate?
It is important to remember when looking on SART databases that the frozen embryo transfer rate reflects ALL frozen embryo transfers. This includes patients who are undergoing a second transfer of “second-best” embryos after undergoing an initial fresh embryo transfer (these patients may be utilizing stored embryos for a second child after a 1st successful IVF and fresh transfer, or may be attempting a second transfer when their initial fresh transfer was not successful). Transfer of “second-best” embryos leads to a lower success rate when compared to FET when all embryos were frozen. SART reporting does not have a location where you can view the so-called “Freeze-all” FETs independently. Intuitively, these success rates are higher, and in various published studies, approximate the success rate of a fresh embryo transfer.
Interestingly, for the first time in 2012, national birth rates per transfer with FET exceeded those with fresh transfer in four of the five age groups, as seen in SART data*. It is important to remember that these numbers can have inherent biases, such as over-representing a group of better prognosis patients that require FET due to OHSS, or patients who have undergone IVF with pre-implantation genetic screening.
How do I know if FET is right for me?
Like most aspects of medicine, recommendations vary in individual patients. The bottom line: although trends show a rise in FET and success, there is not yet any clear choice that maximizes success rates for all patients at all centers. Talk with your doctor about the factors listed above. Together, you can review the pros and cons, the risks and benefits to determine if FET is the right choice for you.
*For more information, refer to the SART database and search pubmed or ask your NRM physician for articles recently published in scientific, peer-reviewed journals, such as:
- Clinical rationale for cryopreservation of entire embryo cohorts in lieu of fresh transfer. Shapiro, Bruce et al. Fertility Sterility July 2014; Vol 102:1.
- Perinatal outcome of singleton siblings born after assisted reproductive technology and spontaneous conception: Danish national sibling-cohort study. Henningsen, AK et al. Fertility Sterility March 2011; 95:3.
- Perinatal outcome of children born after frozen and fresh embryo transfer: the Finnish cohort study 1995-2006. Pelkonen, S et al. Human Reproduction April 2010; 25:4.