A study published in the journal Fertility and Sterility in September of 2017 concluded that “the embryo quality of vitrified oocytes was not impaired… and implantation rates were similar between fresh and vitrified oocytes.” But what does that mean, in laywoman’s terms? Let’s dive in.
First: is this a good study?
When considering the results of any study, it’s super important to make sure it’s a good study, scientifically speaking. Is it large enough? Does it control for outside influences that could affect the results? Do its conclusions align with the conclusions of other, similar studies? (Time to dig that lesson on the scientific method we learned in 4th grade up from the old memory banks!)
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This study had a large sample size, examining 3,794 embryos created from frozen eggs in 631 cycles and 9,935 embryos created from fresh eggs in 1,359 cycles over a period of two years. It’s a very recent study, meaning that all the eggs frozen were done so using modern technology (vitrification), as opposed to older technology (slow freezing). That’s great, because it means the results are applicable to eggs frozen today.
Learn more about vitrification vs. slow freezing.
The main limitation of the study is that it’s retrospective, meaning that it looked backwards at embryos created before the study began, which doesn’t allow researchers to control for every variable. However, because all of the eggs were frozen and thawed, and all the embryos created and transferred, at the same facility, researchers know that comparable laboratory and clinical techniques were used in each case. Overall, this is a very good study.
How was the study performed?
In order to specifically study the possible impact of freezing eggs before using them, researchers at Instituto Valenciano de Infertilidad Valencia (IVI) compared the embryo development, quality, and pregnancy rates for embryos created from donor eggs, obtained from women who were not known to be infertile, that were frozen before using them with embryos created from donor eggs that were never frozen.
In order to do so, they examined the images captured on an “Embryoscope,” a special incubator that records the development of the embryos in a time-lapse fashion, for over 12,000 embryos created over a period of two years. They used these images to determine how long it took for the cells in the embryos to divide after fertilization and to grade the embryo quality, based on their shape and evolution. Finally, they followed the embryos through the transfer step—when they’re inserted back into the uterus in hopes of a pregnancy—to determine what percentage of the embryos implanted and resulted in a live birth.
The results: embryos created from frozen eggs developed slightly more slowly, but embryo quality and pregnancy rates were essentially equivalent to fresh eggs.
The researchers found that embryos created from frozen eggs took, on average, an additional hour to divide and develop. However, that delay did not have an effect on the quality of the embryos, as the percentage of high-grade embryos and low-grade embryos was the same for frozen eggs as it was for fresh.
The implantation rate, defined as the number of gestational sacs (early-stage placentas) detected on an ultrasound, was essentially unchanged between the frozen-egg group and the fresh-egg group. And importantly, there was no statistically significant difference between the live birth rates, based on the journey of the egg.
Embryo transfer type
Live birth rate
A note: researchers examined cycles that used both day 3 embryo transfer, in which developing embryos are transferred on the third day after fertilization, and blastocyst embryo transfer, in which the embryos are transferred after reaching the blastocyst stage, which is typically the fifth or sixth day after fertilization. In the past, day 3 embryos were routinely transferred; today, blastocyst transfer is more common.
Blastocysts are more developed, and therefore it’s more clear to embryologists whether or not the embryo is likely to implant and succeed in the uterus. For that reason, blastocyst transfer cycles typically involve fewer embryos with a higher success rate. It’s important to compare apples to apples; in other words, to compare the success rates for fresh- and frozen-egg embryos based on the type of embryo transfer used in that cycle.
We can conclude from this study that the process of vitrification (flash freezing) doesn’t affect the quality of the embryo that the egg might become later, nor does it affect a woman’s chance of having a baby from those eggs. This is great news for women considering egg freezing; it means that your frozen eggs will give you the same chance of pregnancy when you thaw them later as they would if you used them at the time they were frozen.
Does other research support this conclusion?
Yes! One of the most important things to consider about a scientific study is whether or not its results are reproducible—and this isn’t the first research to conclude that frozen eggs are just as viable as fresh eggs.
The conclusions of this study are supported by the results of an earlier study by the same IVI team, published in 2010. In that study (a randomized controlled trial, the most convincing and reliable kind of study, scientifically speaking) researchers examined the success rates of over 600 women undergoing in vitro fertilization using both fresh and frozen eggs from donors. The result: ongoing pregnancy rates were similar between the group using frozen eggs and the group using fresh, confirming the “non-inferiority,” as the researchers put it, of frozen eggs.
The age of the egg is still important!
It’s key to note that the eggs used in these studies came from egg donors, who are typically healthy and relatively young women. We know that success rates for egg freezing—and all fertility treatments—are extremely dependent on age, so it’s important to remember that you can’t generalize these success rates to all women who might freeze their eggs.
Learn more about egg freezing success rates.