How Are Your Embryos Frozen?


The goal of an egg retrieval is to create as many viable embryos as possible. The reality is that even if an egg retrieval leads to 30 viable embryos, a standard embryo transfer into an intended mother or surrogate consists of just one or two embryos.

Following an egg retrieval, lab technicians will attempt to fertilize the eggs with sperm from either an intended father or sperm donor.  It is common for some eggs to not fertilize. It is also common 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 that are not used at the time of the embryo transfer are frozen and kept for potential transfers in the future.

What is embryo freezing? How does it work?

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˚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˚ F. At this point, it is considered safe to plunge the embryo from the vial into the liquid nitrogen.

How are frozen embryos stored and monitored?

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 -320˚ F! As the liquid nitrogen leaks into the tank, the supply will eventually deplete and force lab technicians to refill the tanks once every several months in order to ensure a consistent deep freeze. As long as a consistent temperature and steady environment are maintained, science suggests that frozen embryos may remain viable indefinitely.

How successful is embryo freezing?

According to a study in 2016, the success rates of using frozen embryos are similar to the success rates of using fresh embryos. As more data is collected over time, the statistics will continue to increase in accuracy.

What are the costs for keeping embryos in frozen storage?

Like any service, the cost can vary greatly among providers. But typically, yearly storage will cost hundreds of dollars while thawing and transferring an embryo will cost thousands.

How long can embryos be frozen?

Because this science has only been around for a few decades, the maximum storage length is unknown. It’s unclear how long frozen eggs last, but there are successful cases of embryo transfers that took place between 10-15 years after the embryos were frozen.

When an embryo is desired for implantation, the freezing 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˚F degrees while it awaits transfer. An embryo can sustain viability for roughly one day in this phase.

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