Science fiction has long been fascinated by the idea of cryopreservation, of freezing a human being so that they can wake up, unchanged, in the future.

While medical science hasn’t perfected cryofreezing for fully grown humans, cryopreservation of embryos is becoming a critical technology that has had a positive impact on the success rate for patients seeking to grow their families.

Cryopreservation – Slow Freezing vs. Vitrification

Slow Freezing

In the past, embryos were cryopreserved using a method called slow freezing. Embryos were run through different chemical solutions to remove water from the cells and replace it with cryoprotectant.

The embryos would then be stored in cryopreservation tubes, and placed in special freezers. These freezers would slowly cool the embryos to -35 degrees Celsius using liquid nitrogen. The cells would then be stored in liquid nitrogen (-196 degrees Celsius). At that extremely cold temperature, cellular activity is essentially brought to a halt, allowing the embryos to remain viable indefinitely.

To understand the problem with this method, it’s helpful to think of an ice cube. When you freeze water in an ice cube tray, you probably notice that the ice cube is slightly larger than the amount of water that you put in. This is because the molecular structure of a water molecule leaves lots of space between molecules when it freezes. That’s why ice takes up more space than liquid water.

The same principle applies to freezing cells which (like most human cells) are mostly made of water. This expansion can damage a cell if it’s kept frozen for too long, and it can also damage the structure of the embryo when it is thawed.


With vitrification, the water molecules in an embryo are removed and replaced with cryoprotectant solution. The embryos are then plunged directly into super-cold liquid nitrogen which “flash freezes” them at a rate of thousands of degrees per minute. The speed at which the embryos are frozen prevents the water molecules from expanding. Instead, the embryos enter what is called a commonly called a “glass” state. This state preserves the embryos and decreases the degeneration of cells when they’re thawed for the embryo transfer.

If slow freezing is like an ice cube, vitrification is like freeze drying. Everything is preserved perfectly. This allows the frozen embryos to stay frozen longer and makes it easier to thaw the embryo.

As its effectiveness has been proven, and as it has become easier to use, vitrification is quickly becoming the new normal. In fact, it was deemed “non-experimental” by the American Society for Reproductive Medicine in 2012.

Why Freeze?

There are two key benefits to cryopreservation.

Improve Embryo Transfer

While the process of IVF is not harmful to a patient, the hormones used can affect how receptive a woman’s womb will be to a transferred embryo. Freezing embryos allows us to extract the eggs, fertilize them, and store them.

Then we can wait until the woman’s hormonal balance is optimal for an embryo, and insert at the ideal time in her cycle.

The ability to freeze embryos also makes it easier to perform Pre-implantation Genetic Screening to determine which embryos are most likely to succeed.

Family Building

Another benefit of embryo freezing is that it allows patients to store embryos for as long as they need to.

Remember, the sooner that a patient can extract her eggs, the more likely they are to be viable. For women in their late 30s, the ability to extract multiple eggs to create embryos for future use helps ensure that she will have healthy embryos for the future. It may also allow her to avoid the need for donor eggs.

A woman can now harvest her eggs in her late 30s, and later have healthy children through her early-to-mid 40s.

At OC Fertility, the use of freezing technologies has allowed us to achieve an average success rate of 70% for women ages 35-44, all using non-donor eggs.

As technologies improve, we expect our success rates to only improve as the technologies become more affordable.

If you’d like to understand more about how we combine freezing with IVF, we invite you to download our Easy Guide to Understanding IVF.

If you have questions, or if you’d like to learn more about me or my practice, OC Fertility, schedule your new patient consultation today or call us at (949) 706-2229.