Thawing Fears: Why Today’s Embryo Freezing is Safer Than Ever

In this episode of Taco Bout Fertility Tuesday, Dr. Mark Amols dives deep into the often-anxious topic of embryo thawing, addressing the common fears patients feel about their embryos surviving the thaw. Dr. Amols takes listeners on a journey through the history of embryo freezing—from early techniques like slow freezing at the pronuclear stage to today’s advanced vitrification process.

He explains why ice crystal formation was once a significant challenge, using the relatable example of a soda can exploding in the freezer, and how modern science has effectively reduced these risks. With advancements in vitrification, survival rates for thawed embryos now exceed 95%—sometimes even reaching close to 99% at top clinics—providing patients with confidence and reassurance.

Join us as Dr. Amols demystifies the process, explaining the science of hydrogen bonds and crystal formation, the introduction of cryoprotectants, and the promising future of automated cryopreservation. By the end of this episode, you’ll understand why embryo thawing is safer than ever and how far the technology has come to ensure better outcomes for IVF patients. Perfect for anyone gearing up for an embryo transfer or those curious about the science behind it all.

Thanks for tuning in to another episode of 'Taco Bout Fertility Tuesday' with Dr. Mark Amols. If you found this episode insightful, please share it with friends and family who might benefit from our discussion. Remember, your feedback is invaluable to us – leave us a review on Apple Podcasts, Spotify, or your preferred listening platform.

Stay connected with us for updates and fertility tips – follow us on Facebook. For more resources and information, visit our website at www.NewDirectionFertility.com.

Have a question or a topic you'd like us to cover? We'd love to hear from you! Reach out to us at TBFT@NewDirectionFertility.com.

Join us next Tuesday for more discussions on fertility, where we blend medical expertise with a touch of humor to make complex topics accessible and engaging. Until then, keep the conversation going and remember: understanding your fertility is a journey we're on together.

Transcript

0:01

Today we talk about thawing your fears. Why embryo thawing is safer than ever, thanks to advances that turn old worries into confidence. I'm, Dr. Mark Amels, and this is Taco about Fertility Tuesday. If you've been through IVF and you've had embryos that are frozen, then you have gone through the fear of wondering if your embryo will thaw perfectly. It feels like one of those make or break moments. Now, technically it is a make or break moment, because if your embryo does not survive the thaw and you only have one embryo, it's true. But the question is, is it something you really need to worry about? And I can tell you right now in the beginning of this podcast, it's not something you really need to worry about. That doesn't mean it can't happen. Your embryo can die in the thought process, but it's actually very, very rare. The question is, then why are we so worried about it? And to understand that, we need to go back to the early 2000s. Ah, yes, the early 2000s, the decade that brought us reality TV, such as my favorite, the American Idol, the first Harry Potter film, and what likely led to the podcast Boom. The ipod. For the reproductive world in the early 2000s, the predominant method for embryo crowd preservation was slow freezing at the pronuclear stage. When an egg is fertilized with a sperm, the first development of the embryo is called the pronuclear stage. And what's unique about this stage is it's going to have a little bit less water than at, let's say, the blastocyst stage, when there's tons of water in the cells. And so at this time, we would freeze embryos at this pronuclear stage. And when we thawed them, the survival rate was around 65 to 70%. Now, keep in mind, this is an average thaw rate when it comes to survival. There were some clinics, such as the one I worked at with Mayo Clinic, that was as high as 85, 90%. But the point of this is, this is where it all started. This is why people were afraid of embryos not surviving the thaw. It's because they actually didn't survive the thaw. In the past, one of the biggest issues was with slow freezing, you would very slowly keep reducing the temperature to basically freeze the embryo. But you didn't want to develop ice crystals. Those would damage the embryos. And so you would add things like cryoprotectants to reduce the formation of those ice crystals and slowly lowering the temperature. Let's Talk about this for a second. One of the very unique things about water is that when you freeze it, it expands. And the reason for this is due to the hydrogen bonds in the water molecule. These hydrogen bonds cause the water to line up in a way that creates a crystal lattice formation that ends up being larger than the molecules would be in a, fluid. And so if you get larger, but you keep the same mass, you have less density. This is one of the reasons ice floats. Many of have seen this phenomenon when we leave our soda in the freezer, as you're aware and probably have confronted before, is that if you leave the soda in too long, it can expand and explode. And then you have frozen soda everywhere in your freezer. Well, the same thing happens to cells and it can cause the cells to die. So there are a couple ways that you can do to reduce this. The first thing you can do is reduce the fluid inside the cells by dehydrating it. The second thing you can do is you can actually prevent the water molecules from lining up and creating that lattice structure by interrupting them. We call this a colligative property. And so we see this all the time. When you see, let's say, ice on the ground, you go and you put salt on it and the ice starts to melt. Now why is that? Well, the salt interferes with the water molecules. And so what it does is it makes it harder for those water molecules to freeze and get in that lattice structure, causing the temperature to have to be colder to cause it to become a solid. So essentially at the same temperature instead become a, solid and now will start to melt. The same property is used to actually cause water to boil at a higher temperature, because by putting salt in it, you interrupt those molecules. And now it takes higher heat temperature to get a higher vapor pressure to be able to make it boil. If you live in high altitudes, you probably have used this before. That's because the, vapor pressure is lower there and water boils at a lower temperature. To get the water to boil a higher temperature, you add the salt, making it increase the temperature for the water then to boil. So the purpose of that was to explain that when they were trying to freeze embryos, they had to find a way to stop the crystal formation. And they did that by dehydrating it and by adding cryoprotectants to interfere with the freezing process and doing it very slowly. That is the colligative property of those molecules. Now, in the mid to late 2000s, this is when we started freezing blastocyst and this is because one of the difficulties of freezing a pronuclear is that when you go to do a transfer, you have to then thaw the pronuclear embryo and then grow it out for three to five days before you can do a transfer. The issue with this is you didn't know how many embryos you had to thaw to get to day three or day five. So potentially, you would have to thaw five embryos to hopefully have one or two blastocysts to transfer in those five days. So it became very difficult because if anything happened that day, it's not like you could just thaw another embryo. It took days for those embryos to make it to that stage. Now, at this point, they were getting better because in the past, we couldn't even make blastocyst freeze and have them survive at a good rate. But now we were getting to the point of around 70, maybe 75% survival rate. That's still not great. One out of four. Who wants to take those type of chances? And so it wasn't until about 2010s when we started getting into vitrification. This is really when everything change. Vitrification is when there's a flash freeze. When we talk about freeze here, it's a little bit different. When something becomes a solid, it has a uniform structure. But when something becomes a solid and it doesn't have a uniform structure, we don't really call it a solid. We call it a liquid. And an example of this would be glass. Glass itself is not a solid. That is because if you look at its structure, it does not form a crystalline structure and is not uniform. Therefore, it is a liquid. And the matter of fact, if you give it enough time, glass will start to drip. Well, the same is true with vitrification for embryos. The embryos themselves are not technically frozen to a solid. They're just moving very, very slow. And because of this, this is going to reduce the risk of getting crystal formation and causing then damage to the embryos. Now, using some of those same principles, we are still using cryoprotectants to prevent crystal formation. And we are also increasing the density by deflating the embryo before it is frozen. Now, at this point in, 2010, American Idol was a shell of itself. I wasn't even watching the show anymore. Additionally, there were any more ipods. Now everyone had iPhones. But when it came to vitrification, now our survival rates were easily in the 90s and up to 95%. This changed everything. Now you could go in for a transfer. If Emirates didn't, let's say, survive the thaw, you could thaw another one. You didn't have to thaw multiple embryos, hoping to get a one or two for the transfer. So it changed everything. It allowed us then to have frozen transfers with a lot more security. But 90, 95%, that sounds great, especially when you were starting at 65 to 70% in the early 2000s. But what about the fact that 5 to 10% would still not survive? That's 1 out of 10 or 1 out of 20. And although that's still a lower number, that's pretty significant, and that's why people were fearful. That brings us now to the 2020s, and this is where continued refinement has led to very high survival rates. Pretty much any clinic now has at least a 95% survival rate. And honestly, great clinics probably have it, exceeding 95% or even close to 99% or higher. This has completely changed the landscape. Now most clinics do frozen embryo transfers as their main form of transferring. Very few people do fresh transfers due to advantages that occur with doing frozen transfers. Additionally, we have serial thawing, where if an embryo doesn't survive the thaw, which is not very common, we can now thaw another embryo and have it ready in time for the transfer. So nothing changes. So over the last 20 years, we have gone from just over a, 50% survival rate to now close to 100% survival rate. Although we're not 100%. We're getting close now. It's important to understand embryos sometimes do not survive the thaw for other reasons. For example, we are using things in extreme temperatures. And so sometimes when you go and freeze an embryo on the tip of a tiny little straw, sometimes that straw may pop when you put it into the liquid nitrogen or when you go to warm it up, and potentially that embryo could disintegrate or pop off and fly away. Additionally, sometimes devices can fail. These are in extremely low temperatures, close to 300 degrees negative Celsius. So it's not surprising sometimes how a device can sometimes fail in that situation, and then when they take it out, it can potentially break. But again, these are very far and few for most people. I would say you can feel pretty comfortable that your embryo is going to survive the thaw. Now the question is, why are there differences between clinics? Shouldn't everyone have the same thaw rate? Pretty much now everyone uses some of the same methods. Everyone's using vitrification. And a lot of the companies are the same now. So Most people are using the same devices. There's slight differences between the straws. But in the end, things have become very standardized. As a matter of fact, one of the things we're going to talk about in a little bit here is whether the advancements coming up, but there are differences when it comes to things like which embryos you are freezing. If you are at a clinic that is freezing every embryo, whether they are good, whether they're poor, whether they're fair, you're going to see a difference in the clinic that only freezes good embryos and fair embryos. Those poor quality embryos are not going to survive the thaw as well. And that's because the embryos are already somewhat compromised. It's not that the thaw process caused it to die, it's that the embryo is already not doing well. And then the stress from that caused things to get worse. And that's why the embryo didn't survive the thaw. Usually very good embryos that are high quality have no problem. This is actually one of the benefits of vitrification. In the past, with slow freezing, any stress to the embryo would start propagations of cascades that could lead to harm to the embryo because it thought it was under stress. Now, with vitrification, we're able to freeze it instantly before even some of those cascades could start happening, reducing the stress on the embryo. Now, with every good thing, there's some bad things. So what's the bad about vitrification, the processes we have? Well, the thing is, it's very technical and so it is harder to learn and you have to do it correctly or you can potentially lead to an embryo not surviving the thaw. Although cryoprotectants sound like they're protecting the embryo, which they are, they also are embryo toxic. And so it's very important when you go to thaw the embryo that those have to be removed right away to prevent harm to the embryo. So as you can see, it's a very delicate dance and it has to have some technical skills to be able to do it. And that's what makes it a little bit harder than the old slow freezing technique. But as I mentioned, there are multiple advantages and the most important advantage is the better survival rate. Now, I mentioned what new things are coming up. One of the newest exciting things coming up is automation. The nice thing about automation is now you're not going to have as much difference between different embryologists. The time between each step is going to be exactly the same and we're probably going to get Less variability between clinics, between embryologists when it comes to freezing and thawing rates. Now, one thing to keep in mind is that when someone says the embryo survived the thaw, that doesn't mean the embryo couldn't be harmed by the thaw. There are chances where it may not work, and the embryo could have been, let's say, slightly injured by the thaw, but they're still alive and it survived the thaw. So when I, talk about survival rates, it's very important to understand we're talking about the embryo surviving it, meaning the embryo did not die in the thought process. In the future, when we're able to automate this and we're able to control things carefully, we might even see higher success rates, because then those small stresses during the thawing process that may have been occurring even though they survived the thaw may be less, leading to potentially higher pregnancy rates in the future. In the end, one of the most important things I wanted you to know is that there is a reason why people feared embryos not surviving the thaw. Because this was a real thing. And since I've been practicing, it's only got better and better, and I only see it getting better in the future. But for you at this time, especially if you only have one or two embryos, rarely does an embryo not survive the thaw, and even when it doesn't, rarely is it the thawing process that caused the problem. But usually there's something else that causes, such as a defective device or some other issue. It is extremely rare for embryos not to survive the thaw. And at this point, I'm not saying you don't have to fear it at all, but it should be one of the lowest fears you have. You can always find a terrible story on the Internet about something where someone lost all their embryos, but usually that was due to some unusual circumstance. That is not the norm. And you should feel confident going into your embryo transfer that you're going to have an embryo to transfer and that usually everything just goes fine. Maybe you're just about ready to undergo a transfer. Maybe you have a friend who's going to be undergoing a transfer. The good thing is you're able to at least learn that you don't have to be as fearful now that most likely the embryo will thaw perfectly without any issues, with a, higher than probably 95% chance, and specifically at some clicks, even as high as 99% and higher. I hope this podcast helped you and helped reduce some of those fears. If you like this podcast, as I always say, and you love it. Tell a friend about it, give us a five star review on your favorite medium. But most of all, keep coming back. I look forward to talking again next week on Taco Bell, Fertility Tuesday.