How Are Embryos Frozen?

Egg retrieval, embryo creation, embryo transfer. These are the parts of the embryo process that most surrogates are familiar with. It is not at all uncommon for more embryos to be viable for use than are needed. The result is that these embryos are often frozen and kept for potential future transfers. Understanding what goes into the freezing of an embryo can be awe-inspiring.

To get things started, you should understand the exact conditions an embryo needs to thrive and maintain viability once frozen. 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 it will eventually become a depleted supply, 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.

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 straw-like vial and into the liquid nitrogen.

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 to warm up to a consistent 98.6 degrees while it waits transfer. An embryo can sustain viability for roughly one day in this phase.

Kim Bergman, PhD, a licensed psychologist of 26 years, has specialized in the area of gay and lesbian parenting, parenting by choice and third party assisted reproduction for over two decades. Dr. Bergman has created a comprehensive psychological screening, support and monitoring process for intended parents, surrogates and donors. She is the co-owner of Growing Generations and is a member of the American Society for Reproductive Medicine, the American Psychological Association, the Los Angeles County Psychological Association, the Lesbian and Gay Psychotherapy Association, and the Gay and Lesbian Medical Association. She is on the national Emeritus board of the Family Equality Council. Dr. Bergman writes, teaches and speaks extensively on parenting by choice. Along with co-authors, she published “Gay Men Who Become Fathers via Surrogacy: The Transition to Parenthood” (Journal of GLBT Family Studies, April 2010). Dr. Bergman’s is the author of the book, Your Future Family: The Essential Guide to Assisted Reproduction (Conari Press 2019) as well as the children's book You Began as a Wish (Independent Press 2019). Dr. Bergman created her own family using third party assisted reproduction and she lives with her wife of 35 years. She has two adult daughters.