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How long can your embryos remain frozen? Does it change the success rates?
—Eggs on Ice
You may have seen the recent headline touting a baby born from an embryo frozen 30 years ago, when the parents were actually toddlers! This extraordinary example (this couple conceived with embryo adoption) shows us that embryos can be frozen for decades and still result in a live birth.
Multiple studies have looked at the length of time an embryo is frozen and success rates with varying results. A retrospective cohort study published in 2022 showed live birth rates peaking around three to four months after the initial IVF cycle and then a gradual decline in pregnancy rates after six months. However, other studies showed no differences in success rates with the amount of time the embryo was frozen. It is important to note that most of these studies were either a retrospective cohort study themselves or a meta-analysis of these types of studies.
Why are the types of studies available important here? One problem with a retrospective cohort study is that it can reduce the researcher’s ability to control for confounding and bias. These studies also rely on accurate records to review and may not account for the differences across people included in the study that could influence the outcome. The rationale behind the time course from retrieval to transfer can vary greatly amongst patients, from some needing surgery for endometriosis to others timing a potential birth with their career trajectory. It is harder to establish a cause-and-effect relationship from cohort studies. Parsing out the potential consequences of cryoprotectants on embryos stored long-term versus the individual factors of each patient that influence embryo transfer success rates makes it hard for us to draw a definitive conclusion here.
We do know the freezing method influences how likely an embryo is to survive the thawing process. Vitrification, or flash freezing, is the method that is currently used to freeze embryos. This method was first introduced in 2006, and most clinics had adopted it by the early 2010s. Vitrification has a very high success rate in terms of the embryo surviving the thaw and creating a pregnancy. The exact rates are dependent on the individual lab, but most have success rates approaching 100%. Prior to the development of vitrification, the “slow freeze” method was used, which had a greater chance of an embryo not surviving the thawing process. However, if you are doing IVF at this moment, vitrification will be used.
The overwhelming majority of research shows that the length of time the embryo is frozen does not impact transfer success rate, regardless of the freezing method used. Cases from couples undergoing embryo donation cycles show us that a baby can be born from an embryo frozen for several decades.
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