By Corey Burke, B.S., C.L.S.
Embryologist & Andrologist
Laboratory Supervisor

I am often asked: “How long do frozen embryos last?” The answer to this question is a little more complicated than simply replying with a specific number of years because several factors influence the answer.

The first successful pregnancy from a cryopreserved (frozen) embryo occurred in 1983 (Trounson A, Mohr L. 1983). Shortly thereafter, embryo cryopreservation became commonplace, with millions of embryos cryopreserved over the past few decades. Recently, a live birth was reported from an embryo cryopreserved for 20 years (Dowling-Lacy D, et al. 2011). There is, however, uncertainty regarding the health of frozen embryos over time. Since embryologists have only been cryopreserving embryos for slightly less than 30 years, it is impossible to know right now if extended storage times much beyond two decades will influence embryo survival and pregnancy rates.

A study that is the largest of its kind was recently published in March 2011. It examined approximately 12,000 cryopreserved embryos, and determined that the length of storage time did not influence post thaw survival rates or the actual pregnancy outcomes (Riggs R, et al. 2010). The embryos in this study had been frozen up to nine years.

The technique for freezing embryos in liquid nitrogen has changed over time. The original method was known as a “slow freeze” technique, where the temperature of the embryos slowly dropped over a couple of hours. Vitrification, a newer method, freezes the embryos in a minute fraction of a second. While survival rates for embryos frozen through vitrification seem higher, it is uncertain if vitrified embryos will result in more pregnancies (Khoury C, et al. 2010). My guess is there will actually be little difference between the two freezing methods since the embryos end up in the same suspended animation of liquid nitrogen at -196°C (-321° F). If the embryo survives the thaw, pregnancy rates may not depend on the duration of time it was frozen or the technique used to freeze it.

While there clearly are many variables that influence the health and implantation rate of an embryo, the length of storage is probably not one of them. Data still needs to be collected but the information we have so far is encouraging.

As time marches forward, I expect additional cases to be reported that will lengthen the 20-year cryopreservation “ceiling” to decades longer. Amazingly, it is estimated that mammalian cells will last for hundreds or even thousands of years when frozen in liquid nitrogen (Mazur P. 1980). If this is true, these frozen embryos will certainly outlast all of us.

References:

Dowling-Lacey D, Mayer JF, Jones E, Bocca S, Stadtmauer L, Oehninger S. Live birth from a frozen-thawed pronuclear stage embryo almost 20 years after its cryopreservation. Fertil Steril. 2011 Mar 1;95(3):1120.e1-3.

Khoury C, Fredrick J, Behr B, Potter D. A Comparison of Blastocyst Slow Freeze and Vitrification in Frozen Blastocyst Transfer. Fertil Steril. 2010 Mar;93(5): S14-15

Mazur P. Limits to life at low temperatures and at reduced water contents and water activities. Orig Life 1980;10:137–59.

Riggs R, Mayer J, Dowling-Lacey D, Chi TF, Jones E, Oehninger S. Does storage time influence postthaw survival and pregnancy outcome? An analysis of 11,768 cryopreserved human embryos. Fertil Steril. 2010 Jan;93(1):109-15.

Trounson A, Mohr L. Human pregnancy following cryopreservation, thawing and transfer of an eight-cell embryo. Nature 1983;305:707–9.