When you create embryos using IVF, you and your doctor will decide whether to transfer an embryo in that same cycle or to freeze your embryos and transfer one in a later cycle. These are referred to as a fresh transfer (the former) and a frozen embryo transfer (FET—the latter). How does this get decided and is one better than the other? Let’s talk about the differences between a fresh embryo transfer and a frozen embryo transfer and which one may be right for you.
What happens in a fresh embryo transfer?
With a fresh transfer, the egg retrieval and embryo transfer happen in the same cycle. An egg retrieval commonly happens after about nine to 14 days of stimulation medications. You will be monitored throughout the stimulation and your clinic team will keep track of the follicles (number and size), endometrium (thickness and pattern) and bloodwork (mainly estrogen, LH and progesterone). After the trigger medications, you’ll do the egg retrieval. Once eggs are retrieved, and confirmed to be mature, then sperm is introduced in hopes that fertilization takes place. An egg that successfully becomes fertilized will start to develop into an embryo, and the embryologist will monitor its development over the next few days. Embryos are most commonly transferred when they are in the blastocyst stage (typically, day five, post-retrieval but many viable embryos might take five to seven days to become a blastocyst). In specific cases your doctor might suggest transferring the embryos back at an earlier stage (like day two or three post-retrieval). This means that the embryo transfer can be as early as three days post-retrieval, provided everything else is looking good.
Are there benefits to a fresh transfer?
Doing a fresh transfer means not having to wait another month or cycle for the embryo transfer, and not having to take hormonal medications for an additional cycle (if your FET protocol requires further medications such as estrogen and progesterone). This time-saving factor is typically a strong motivator for many people when it comes to deciding on a fresh transfer. There is also less risk of the embryo becoming damaged because it’s not being put through a freeze and thaw process. Rest assured that in most IVF labs, it is rare for the embryo to not survive the freezing and thawing process, but it remains a consideration and should be seen as an additional stressor for the embryo.
Why might you not do a fresh transfer?
OHSS (Ovarian Hyperstimulation Syndrome)
Patients with a high ovarian reserve will respond well to fertility medications. This means that many of their follicles within their ovaries will grow, and since each produces estrogen, the estrogen levels can get really high. This creates a risk of developing ovarian hyperstimulation syndrome (OHSS). In mild cases of OHSS it’s common to feel bloated, nauseous and have lower abdominal discomfort. But severe cases can be life-threatening and affect blood pressure as well as the function of key organ systems like the kidneys, liver and lungs (keep in mind this only happens in about 0.5 to five percent of IVF cycles).
What happens in a frozen embryo transfer?
Generally, embryos that develop into blastocysts can be frozen and stored. But even earlier stage embryos (day two or three embryos) can be frozen if that’s your personalized treatment plan. The frozen embryos can then be thawed (typically one at a time) and transferred in a later cycle.
There are different protocols for FET cycles including:
- Natural FET: This relies on your natural cycle. A growing follicle will release estrogen which thickens the uterine lining (a.k.a., the endometrium), and once you ovulate, the corpus luteum will release progesterone to transform the lining in preparation for implantation. In a natural FET you will be monitored with ultrasound and bloodwork to track the growing follicle and lining thickness. Each clinic has their own policy about how to synchronize the embryo thawing with your cycle, but it’s generally five to seven days after the LH surge. There are natural FET cycles that will not involve any additional medications (pure cycles) and there are natural FET cycles that might have some additional medications such as a trigger medication or progesterone support in the luteal phase (modified cycles).
- Medicated FET: This typically refers to treatment cycles where the estrogen and progesterone is provided to the patient. The first half of the cycle will be estrogen only to help the lining thicken, and then progesterone is prescribed to support the lining.
We recommend you speak to your clinician to personalize your FET protocol, but overall you can be reassured there is no clear difference in pregnancy success rates between doing a natural versus medicated FET. The goal is to have a receptive uterine lining that is ready and thick enough for implantation to occur and sometimes we need supplemental hormones to get there.
Are there benefits to doing a frozen transfer?
One reason to choose a frozen over a fresh transfer is if you’re interested in preimplantation genetic testing (PGT-A). This is where the embryos are tested to see if they have the correct number of chromosomes before they are transferred. This helps narrow down which embryos to try and transfer first, for a greater chance of implantation and survival. However, studies show that PGT-A is most helpful in those 38 or older (and in some cases of recurrent pregnancy loss), as this is when aneuploidy rates (having an abnormal number of chromosomes) tend to be significantly higher.
Which has a better success rate, fresh or frozen transfer?
Studies have shown that in ovulatory women undergoing IVF, there is no difference in implantation, clinical pregnancy and live birth rates between fresh and frozen transfers, but the risk of OHSS is lower in frozen transfer cycles (one study found a difference of 0.6 percent vs. 2 percent). In one study of women with PCOS, the live birth rate was higher (49 percent vs. 42 percent) in those who did a day-three frozen embryo transfer compared to those who had a day-three fresh transfer. Each case and each protocol will be different, so it’s important to take that individuality into consideration and talk to your doctor about your options and any associated risks.
Both fresh and frozen embryo transfers should be considered viable options. There are several reasons to strongly consider freezing your embryos including to minimize the risk of OHSS or to be able to screen the embryos with PGT-A testing. Even with FET cycles there are opportunities to minimize medications with a more natural approach. It’s important to discuss your personal clinical situation with your fertility doctor.