How Do Frozen Embryos Live?

In order to preserve embryos through freezing or cryopreservation, they must be stored at extremely low temperatures, generally between -196°C and -320°C. At these temperatures, cellular metabolism is stopped, which allows the embryos to remain viable indefinitely.

To reach this preservation temperature, embryos are submerged in liquid nitrogen (-196°C) using a technique called vitrification. This ultra-rapid freezing method prevents the formation of ice crystals inside the cells, protecting their integrity and increasing survival rates after thawing.

How Is Vitrification Performed? 

The embryos are cultured in a specialized medium until they reach the blastocyst stage, which generally occurs between days 5 and 6 of development.

Next, the embryos are exposed to cryoprotectants, which are substances that replace the water inside the cells to prevent the formation of ice crystals that could damage cellular structures.

Once ready, the embryos are submerged in liquid nitrogen for ultra-rapid freezing and stored in nitrogen tanks or specialized cryopreservation chambers.

How Long Can Embryos Remain Frozen? 

Embryos can remain cryopreserved indefinitely without losing viability. In Spain, current legislation allows embryos to remain frozen for as long as the patients wish, provided that consent is periodically renewed.

What Happens to Embryos After Thawing? 

Embryos are thawed when they are to be used and transferred to the patient’s uterus, a procedure known as frozen embryo transfer (FET).

The survival rate of vitrified embryos after thawing is between 90% and 95%, which means that some embryos may not survive the process. For this reason, it is recommended to cryopreserve several embryos to increase the chances of treatment success.

Do Frozen Embryos Have the Same Success Rate as Fresh Ones? 

Current studies indicate that the implantation rates of frozen embryos are similar to those of fresh embryos.

What Options Are Available for Frozen Embryos?

• Use in assisted reproduction treatments.
• Preservation for future attempts, allowing for postponed motherhood or new treatment planning.
• Donation to other couples who cannot conceive.
• Donation for scientific research to contribute to advances in assisted reproduction knowledge.
• Destruction, in cases where patients decide not to use or donate them, and when meeting the criteria for destruction.