Egg freezing and thawing

This post goes over egg freezing and thawing, with a focus on survival rates of thawed eggs, whether egg freezing leads to lower embryo quality, differences in pregnancy rates, and how many frozen eggs are needed for a live birth.

Survival of the egg after freezing and thawing has been a concern. Compared to blastocysts, or even cleavage stage embryos, the egg is a single (and large) cell that is prone to ice crystal formation and damage.

With “vitrification” the egg (or embryo!) is frozen very quickly to -196*C in liquid nitrogen. Because it is done so quickly and using special freezing reagents, there is very little ice formation.

The use of vitrification (as opposed to the older “slow freezing” technology) has improved survival over the last 15 years. Survival is between 65-99% with vitrification compared to 55-90% with slow freeze; the benchmark for oocyte survival with vitrification is >80% (Alpha scientists in reproductive medicine, 2012).

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Table of Contents

Does egg freezing increase aneuploidy?

The egg is a single cell that contains a lot of important components mixed up as a kind of “soup” in its cytoplasm.

One critical component is the meiotic spindle. This is a delicate structure in the egg that is responsible for separating the egg’s chromosomes. If this structure is damaged by vitrification then this might lead to chromosomes not being separated properly and this can cause aneuploidy.

When comparing aneuploidy of blastocysts that develop from fresh vs vitrified eggs, there is no difference (Forman et al. 2012) . So, at least based on this study, this tells us that vitrification is not damaging the sensitive meiotic spindle structure and causing aneuploidy.

Does egg freezing lead to lower embryo quality?

A good way to see if egg freezing affects embryo quality is to see if there are differences in pregnancy outcomes. A study by Gursu et al (2022) found no difference in pregnancy outcomes when using fresh or frozen donor eggs.

They also found no differences in blastocyst conversion – the number of embryos that went on to form blastocysts. This is a good sign for embryo quality!

You can read this study in detail in my post No differences in IVF outcomes when using fresh or frozen donor eggs.

An older study by Forman et al. 2012 compared vitrified eggs vs fresh eggs:

Lower fertilization with vitrified eggs (77.9% vs 90.5%)
Fewer embryos made it to the cleavage stage (90.9% vs 99.2%)
Fewer embryos made it to blastocyst (34.8% vs 50.8%)
Fewer fair/good quality blastocysts (22.1% vs 45.9%)
No difference in aneuploidy (29.1% vs 26.4%)
No difference in ongoing pregnancy rate (53.9% vs 57.7%)

While this study shows lower embryo quality, it’s reassuring that the embryos had a similar rate of aneuploidy and ongoing pregnancy.

What is the survival rate of frozen eggs that are thawed?

There are different factors that can affect egg survival after thawing. Mainly technical issues, egg quality, and female age.

Not all labs/embryologists have the technical skill to freeze and thaw eggs. In some labs, or in the hands of some embryologists, egg survival is lower compared to others, so you should ask your clinic if these are the kinds of survival rates they’re seeing with their donor eggs.

Not all eggs will tolerate freezing! In a 2015 paper by Cobo et al., they found that out of 3146 egg donation cycles from egg donors, 1.4% (or 45 cycles) had 0% survival. They also found that if a cycle had poor survival, then this was more likely to occur in future cycles. So some egg donors are simply not suitable for egg freezing.

Another study (Coello et al. 2019) looked at how abnormal egg quality (based on abnormal zonas/shapes/perivitelline space/granularity/smooth endoplasmic reticulum) affected different outcomes:

No difference in egg survival after thaw in abnormal vs normal eggs
No difference in blastocyst formation rates
No differences in good quality blastocysts

So what does have an impact?

Cobo et al. (2016) found that age had a strong impact on egg thaw survival:

<29: 94.5% of eggs survived
30-34: 96.1%
35-39: 85.8%
>40: 81.3%

When freezing eggs, how many do we need to achieve a live birth?

This question was addressed in Cobo et al. (2018) and it depends on the woman’s age.

The stats below are for cumulative live birth rates. This is the % of women who had a live birth after transferring as many embryos as it took to have a live birth (from the eggs they had thawed).

For those <35:

5 eggs has a cumulative live birth rate of 15.8%
8 eggs is 32.0%
10 eggs is 42.8%
15 eggs is 69.8%
20 eggs is 77.6%
24 is 94.4%

For women >35:

5 eggs has a cumulative live birth rate of 5.9%
8 eggs is 17.3%
10 eggs is 25.2%
15 eggs is 38.8%
20 eggs is 49.6%

So live birth rates can be very high – as long as you have enough eggs to get you there!

About Embryoman

Embryoman (Sean Lauber) is a former embryologist and the founder of Remembryo, an IVF research and fertility education website. After working in an IVF lab in the US, he returned to Canada and now focuses on making fertility research more accessible. He holds a Master’s in Immunology and launched Remembryo in 2018 to help patients and professionals make sense of IVF research. Sean shares weekly study updates on Facebook, Instagram, and Reddit regularly. He also answers questions on Reddit or in his private Facebook group.