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Choosing to freeze eggs, sperm or embryos sounds futuristic and costly — not to mention like a science project that can be tricky to understand. But, if you’re considering freezing any of these reproductive components, know that the process and techniques differ based on whether you choose to freeze eggs, sperm, or embryos.

How Freezing Eggs Works

Modern approaches to freezing eggs have drastically improved IVF success rates — and that’s a great thing, considering the time, emotional drain, and cost of undergoing egg freezing. In general, there are two main ways eggs are frozen in the United States: slow freezing and flash freezing.

Early egg freezing, starting around the late 1980s, used a technique called “slow freezing,” where eggs were retrieved, mixed with a protective solution called a cryoprotectant, and gradually frozen over the span of three or more hours. From there, frozen eggs would be kept in storage at super low temperatures (normally around -196 degrees Fahrenheit), waiting for the day they’d be thawed and used.

Unfortunately, the slow freezing process — which is still an option for women who are freezing their eggs — isn’t without fail. Because human eggs have such high water content, freezing can cause damage thanks to the ice crystals that form. Slow freezing often uses less cryoprotectant than modern fast-freezing techniques, leaving eggs more susceptible to frost damage. Studies on slow freezing have found that eggs stored using this method have about a 66% freeze-thaw survival rate.

Around the early 2000s, a fast freezing technique was introduced to the world of oocyte cryopreservation (the scientific term for egg freezing) that involved faster, more efficient, and more reliable technology. Going by several names — fast freezing, flash freezing, or egg vitrification — this newer process freezes eggs within minutes.

To get started, retrieved eggs are soaked in a cryoprotectant solution that dehydrates them and removes much of their water. From there, the eggs are moved to a second kind of freeze-protection solution and soaked for a short time before being divided and collected into storage containers. This process allows several vials of eggs to be kept, ensuring that if something happens to one, not all of your eggs are lost. To get the freezing started, each container of eggs is submerged in liquid nitrogen. After a few seconds, they’re successfully frozen and moved into storage.

Despite the fact that egg freezing technology has improved drastically over time, slow freezing — also called controlled rate or slow programmable freezing — is still an option at some cryopreservation labs. This means that if you feel fast-freezing is the better option for your eggs (and wallet), it’s important to double-check which method your cryobank uses.

How Embryos Are Frozen and Stored

Some individuals or couples choose to freeze embryos instead of eggs or sperm — especially if they’ve already undergone a round of IVF and have leftover embryos that can’t be implanted. When it comes to freezing embryos, cryopreservation labs use the same techniques as freezing eggs — either the slow freeze or flash freeze methods explained above.

Embryos can be frozen and stored at different stages in their development process, so it’s important to determine if the storage lab you’re considering is able to accommodate a range of dates. Because embryos have multiple development stages, they can be frozen at the pronuclei stage (day 1), multicelluar stage (days 2 and 3), or as blastocytes (days 5 and 6). Each and every lab has a preferred approach to freezing embryos based on their experience and these stages, simply because some stages may be easier to freeze or have more successful results — though there’s no hard and fast rule that suggests a day 1 embryo is better or worse than any other stage.

In every embryo freezing scenario, a cryopreservation lab will determine if it’s best to proceed quickly based on the embryo’s current stage, or if the process should hold off for a short period of time to allow the embryo to reach the next stage.

How Sperm Is Frozen

If you or a partner chooses to undergo sperm freezing, you’ll be signing up for a process called semen cryopreservation. This process essentially consists of several types of genetic testing, freezing, and sample semen analysis. These multiple types of testing ensure that the preserved sperm will hold up to freezing, which is helpful in determining how viable they’ll be whenever it’s time for them to be thawed. So far, the longest reported storage of sperm that was viable and successful at fertilizing an egg has been 24 years.

Sperm freezing starts with sample collection. In most cases, an individual or donor’s semen will undergo a set of genetic tests prior to being frozen to identify any potential abnormalities or issues that may impact viability — or cause health issues for potential children. This semen analysis will also provide insight as to the viability and quality of your sperm.

Before heading into storage, the semen sample is then divided into smaller vials so that it can be frozen successfully. During this process, a protective mixture (called a cryoprotectant) is mixed with each sample to help the freezing process and minimize any possible damage during freezing.

From there, the actual freezing process begins, taking only about an hour. Each vial of sperm is slowly frozen in liquid nitrogen before being transferred to a storage tank. Though, one last round of testing will take place before a lab considers the process complete. Around two days after the samples are frozen, a cryo lab will pull one ample to thaw and test. This extra analysis determines how well the sperm has survived freezing and thawing, and is a safeguard to ensure the remaining frozen sperm can be used later one.

Once undergoing the semen cryopreservation process, it’s important for sperm to stay at exceptionally cold temperatures to remain preserved. In most cases, this is about -196 degrees Fahrenheit.

So, why consider freezing sperm? Some may take this route in their reproductive planning due to health conditions. Cancer treatments, including radiation and chemotherapy, can impact sperm motility and viability, so some patients choose to bank sperm as one possible avenue to being fathers in the future. In other situations, men who are considering a vasectomy have frozen their sperm as a backup plan in case they choose to have any (or more) children later in life. It’s also not uncommon for active military members to freeze sperm to ensure they’ll be able to have children in the future regardless of possible military-related injuries or disabilities.

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