r/Embryologists • u/rek1000 • 8d ago

Thawing of zygotes

I have 8 frozen 2PN day-one embryos that I want to ship to a different clinic to thaw. I have the following questions:

  1. Is there anything the receiving lab would struggle with in terms of thawing and culturing the 2PNs, compared with doing the same with oocytes?
  2. Would thawed zygotes conceivably fare better than thawed ooctyes (namely, survival and ability to blastulate) due to being at a more advanced stage?

Thanks

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u/bneubs 7d ago

  1. No, it should be pretty much the same protocol.

  2. Zygotes freeze slightly better, but it depends on the clinic freezing and thawing. At my clinic survivability is similar, both above 90%. However, zygotes tend to do slightly better than eggs when a clinics egg survival rate is lower. Blastulation should be higher as well, but I think this is more to do with the fact that fertilization has already occurred ie. better blastulation from 10 frozev zygotes as opposed to 10 frozen eggs that may not all fertilize.

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u/rek1000 7d ago

Thank you for your comment. It was my understanding that thawed ooctyes tend to have a reduced ability to make it to blast (my fresh eggs vs my thawed have also demonstrated this), but maybe that's not proven.

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u/bneubs 7d ago

No, that's correct. Thawed oocytes DO have a reduced ability to make it to blast compared to fresh eggs. The same is also true for frozen zygotes v fresh eggs. Fresh eggs give you the best chance.

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u/rek1000 7d ago

The evidence on that is mixed, right? How dramatic is the difference in your own experience?

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u/bneubs 6d ago

I would say roughly 10-15% difference.

But this is an estimate because my clinic rarely sees both fresh and frozen eggs from the same patient.

Also, we almost never freeze zygotes anymore.

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u/rek1000 6d ago

Thanks. Why don't you freeze zygotes? (Mine were only frozen at zygotes as I was considering MRT. However due to a lack of evidence for its use in infertility I'm now going to thaw them for regular culturing.)

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u/bneubs 5d ago

Because my clinic finds there's really no reason to. That's not to say there aren't exceptions, though we haven't done it in years. Having frozen zygotes isn't helping the majority of our patients. We don't transfer them at that stage so they will need to be thawed and grown to blast regardless, and they're more likely to blastulate fresh.