During your donation process your medical team will closely monitor your follicle development. Many egg donors become curious about the number of eggs they are able to produce at the culmination of their experience- egg retrieval day. It is often surprising to them to learn that while retrievals of up to 30 eggs are not uncommon, a standard transfer consists of just one or two embryos.
Following your retrieval, lab technicians will attempt to fertilize your eggs with sperm from either a donor or an intended father. It is common for some eggs to fail to fertilize. It is also not uncommon for more embryos to be viable for use than a fertility doctor will need in order to attain pregnancy. The result is often that these embryos are frozen and kept for potential future transfers.
The actual embryo freezing process takes just a matter of hours to complete. The first step involves bathing the embryo in a solution that is similar to sucrose. The solution forces all moisture out of the embryo in order 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! 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.