Embryo freezing is commonly used to preserve embryos that have been produced, but that are not transferred to the uterus. To date, over 150,000 births have occurred from the transfer of previously frozen embryos, some having been frozen for more than two decades. Embryos are frozen using a computer-guided process resulting in a final temperature of -196°C. At this temperature the embryo ceases all metabolic activity until thawed. The thawing process, however, can lead to trauma and damage of the embryos, leading to a loss of viability.
In the past, freezing embryos lowered pregnancy rates by as much as 50%. However, with newer freezing methods such as vitrification, pregnancy rates are much better, approaching those of fresh (never-frozen) embryos. As the technology becomes more precise, these success rates continue to climb, thus making embryo freezing a more appealing and viable part of the field of reproductive medicine.