This blog is part of Fertility FAQ, a series where RMA doctors answer your most pressing questions about treatment.
What is the difference between a frozen or fresh embryo transfer?
Frozen Embryo Transfer (FET) is becoming standard practice in the In Vitro Fertilization (IVF) industry these days. Transfer of cryopreserved embryos optimizes the advantages gained by modern fertility techniques, offering higher pregnancy rates and safer pregnancies.
FET is simple – it is when embryos are cryopreserved (frozen) prior to transfer into a woman’s uterus.
The embryos remain safe and stable in the cryopreserved state for weeks to years. Fresh embryo transfer is the opposite approach when doctors transfer an embryo into a woman’s uterus as soon they feel it is ready for implantation.
While fresh transfers used to be common, advances in technology and reproductive medicine have paved the way for FET, which leads to higher pregnancy success rates.
Why a frozen embryo transfer is preferred
Why? Three reasons.
The first is that FET gives a woman’s body the chance to recover after egg retrieval and before implantation.
The second is that FET allows doctors to sync up the timing between the embryo and the uterus so implantation can be successful.
And the third is that by freezing the embryo, embryologists have time to test it for chromosomal abnormalities prior to transfer.
The benefits of a frozen embryo transfer
Let’s dive into each of those benefits, starting with the first. In order for a woman to grow many eggs during her IVF cycle, she must take ovary-stimulating medications, or hormone injections, for about ten days before her egg retrieval.
While overall safe, in the short term, these medications can make a woman feel, well, hormonal. Many women note bloating and moodiness during their cycles.
A woman’s hormones are at their highest point right after her eggs are retrieved. If she were to become pregnant in that time frame, her hormone levels would soar higher, exacerbating her symptoms and placing her at higher risk of ovarian hyperstimulation syndrome, a serious complication of IVF.
With FET, where doctors wait several weeks after egg retrieval to transfer the embryo, a woman’s body has a chance to recover and return to a natural, baseline state. This helps the expectant mother to feel well during the early days of her pregnancy and may reduce the risk of abnormal placentation.
The second benefit of FET, and one that leads to higher success rates, is something we call uterine receptivity. And all this means is that the uterus is ready to receive the embryo, increasing the chances of implantation and ongoing pregnancy.
The truth is that each cycle, your uterus is only receptive to an embryo for a short window of time, between 24-36 hours. And an embryo can only successfully implant into the uterus once it is mature enough, which is when it reaches the blastocyst stage.
If both embryo and uterus don’t “meet each other” at just the right time, patients miss the chance to become pregnant that cycle.
For example, if a perfectly healthy embryo is growing a bit slowly, it may miss the window of implantation for the uterus. Attempting a fresh transfer of this embryo would require placing the embryos in the uterus prior to the time when the embryo is ready or waiting so long for the embryo to get to its optimal stage that the uterus has now lost some of its receptivity.
FET permits doctors to freeze the embryo at the blastocyst stage and transfer it at just the right time, when the uterus is most receptive to it, increasing the chances of success.
Frozen embryo transfer allows for genetic testing
Finally, those few weeks on ice before transfer give embryologists time to test the embryo for its chromosomal makeup using Preimplantation Genetic Diagnosis (PGD).
This test checks whether an embryo has the right number of chromosomes (46) or too few or too many.
Embryos with the complete set of chromosomes are normal, and lead to successful pregnancies and live births, whereas embryos with a chromosomal imbalance are abnormal and fail to implant, lead to miscarriage, or result in an affected baby.
Once the embryo reaches the blastocyst stage, an embryologist will take a tiny biopsy of the embryo (cells that will become the placenta) before freezing the embryo.
That biopsy sample will then be analyzed for its chromosomal makeup, a process that can take a week or more and would be impossible with fresh embryo transfer. Once the results are back, doctors can thaw and transfer the normal embryo, increasing a woman’s chance of success.
This trio of benefits is why FET is now practiced in the most advanced clinics in the world, with some of the highest success rates.