Fresh vs. frozen
This year, IVF celebrates its 40th anniversary. The technology for freezing (or cryopreserving) extra embryos to delay embryo transfer until a later time has existed for almost as long. The first baby born from transfer of a frozen embryo was reported in 1984.
For years, it was a common view that fresh embryos were superior to frozen, because undergoing freezing and thawing could result in the formation of ice crystals in the embryo and cause cellular damage. However, freezing technology has advanced so dramatically over the last few decades that frozen embryos are now considered equivalent to fresh, and fresh embryo transfer is no longer necessarily the standard of care. In fact, if you are undergoing IVF now, your reproductive endocrinologist (RE) might give you two options:
- having a fresh embryo transfer in the same cycle as your eggs were retrieved,
- freezing all of the embryos, and coming back at a later time for transfer.
Embryos are frozen in solution with cryoprotectants, which are essentially anti-freeze agents that permeate the cells of the embryo and protect them from ice-crystal formation and from damage from the freezing process. Nowadays, most IVF labs freeze embryos after five or six days of development, at what is known as the “blastocyst” stage.
A blastocyst embryo contains over 100 cells and, at this advanced stage, is more resilient to freezing and thawing. The survival rate of a frozen blastocyst embryo is over 95%, which is much higher than when embryos used to be frozen at earlier stages of development.
The second major advancement in freezing technology is our ability to “vitrify” or quick-freeze embryos. When freezing was first developed, it could take hours to bring the temperature of the embryo down from room temperature to -196 degrees Celsius, which is the temperature of liquid nitrogen in which embryos are stored. Nowadays, with newer vitrification techniques and solutions, embryos can be frozen within seconds, and suffer less damage from the process. At -196 degrees Celsius, no biological activity occurs, and frozen embryos have great longevity. The longest an embryo has been frozen before resulting in a live birth was 24 years!
So, to freeze or not to freeze? Here are some scenarios in which your RE might recommend the “freeze-all” option and for you to return at a later cycle for embryo transfer.
You are at risk of developing OHSS.
Ovarian hyperstimulation syndrome (OHSS) is a known risk of IVF stimulation. The medications you’re injected with are intended to encourage many eggs to grow, but in some women, excessive egg development can lead to this condition. If severe, OHSS can cause significant fluid accumulation in the abdomen, kidney damage, breathing difficulty due to fluid around the lungs, and abnormal blood clotting.
One thing that worsens the condition is getting pregnant in the same cycle as the stimulation, because the natural hormones of pregnancy can exacerbate OHSS. Therefore, to prevent OHSS, your RE might recommend a freeze-all if too many eggs are retrieved in your IVF cycle, in order to delay pregnancy until you have recovered.
You are doing PGT.
Preimplantation genetic testing (PGT) is a technology used to test embryo(s) for genetic error before transferring them back to the uterus. It involves growing embryos to the blastocyst stage in the lab, biopsying or removing a few cells from the outer layer of each embryo, and sending the cells for genetic testing, before deciding which embryo is normal enough for transfer.
In most labs, it takes several days to get PGT results back, so embryos have to be frozen while you’re awaiting the results. At a later time, frozen embryos deemed to be normal by genetic testing are then thawed (usually one at a time) for transfer.
There is a problem with your uterine lining.
During your IVF cycle, frequent ultrasounds are performed to monitor egg development and the thickness of your uterine lining. Sometimes, problems are detected in the lining that could interfere with implantation of the embryo. For example, polyps (small overgrowths of tissue within the uterine lining) might be observed, or abnormal fluid might accumulate. In these scenarios, your RE might recommend a freeze-all because the uterine environment is not optimal for embryo transfer. After the IVF cycle, more investigation or treatment might be performed on the uterine lining to optimize the environment before a future transfer of a frozen embryo.
With improved freezing techniques, some patients wonder whether they should decline a fresh transfer and opt to freeze all of their embryos routinely. Although there is some scientific evidence that the hormones produced during IVF stimulation could slightly reduce the ability of the uterus to implant and could affect future placental function, the jury is still out on whether avoiding fresh embryo transfer and freezing all routinely is a good idea. The downsides of freezing all embryos routinely are that not all women can produce enough high-quality blastocyst embryos to freeze, and up to 5% of embryos might not survive the freezing and thawing process.
For now, we recognize the pros and cons of embryo freezing, but we feel very comfortable that pregnancy rates with embryos frozen using today’s technology are equivalent to those with fresh embryos. So, if your RE recommends freezing all of your embryos and coming back another day for them, rest assured that this is commonly done and that your embryos will be well taken care of!