The goal of your egg retrieval is to create as many viable embryos as possible. The reality is that, even if your egg retrieval leads to 30 viable embryos, you’re likely to transfer no more than two. The most common outcome is that many of your “left over” embryos will be frozen for potential future transfers.
The embryo freezing process takes just a matter of hours to complete. The first step involves bathing the embryos in a solution that is similar to sucrose. The solution forces all moisture out of the embryo to prevent ice crystals from forming during freezing. Such crystals would destroy an embryo.
Once bathed and free of all internal moisture, the embryos are transferred into a very thin straw. This straw is then placed into a computerized freezer for the first phase of freezing. This phase takes the embryo from room temperature to roughly -18 degrees F.
Next, depending on how the lab chooses to freeze embryos, a technician may begin a process known as “seeding” the embryo. In this process, the technician will touch cold objects to the straw in order to encourage any potential ice crystals to form outside of the cell structure. Following this, the embryo’s temperature will be slowly reduced at a rate of about half a degree per minute until the embryo reaches a temperature between -30 and -130 degrees. At this point it is considered safe to plunge the embryo from the vial into the liquid nitrogen.
Embryos must be frozen in the presence of liquid nitrogen in order to be kept cold enough for viability. When properly stored in these free standing tanks, embryos are cooled to temperatures of negative 320 degrees Fahrenheit! As the liquid nitrogen leaks into the tank, the supply will eventually deplete, forcing lab technicians to refill the tanks once every several months in order to ensure a consistent deep freeze. So long as a consistent temperature and steady environment are maintained, science suggests that frozen embryos may remain viable indefinitely.
Later, when the embryo is desired for implantation, this process will be slowly reversed. The embryo will be washed of all protectant solution and allowed to warm to room temperature over a period of several hours. Once room temperature is attained, the embryo will be placed inside of an incubator where it will be warmed to a consistent 98.6 degrees while it waits transfer. An embryo can sustain viability for roughly one day in this phase.