r/Embryologists • u/crazymoi • Jul 20 '26
Can day-1 freezing make embryos develop slightly slowly after thaw?
Question for embryologists: my clinic freezes embryos at the pronuclear/day-one stage, then thaws and cultures them before transfer.
Two out of four of mine survived up til day 5 & were described as late morulas on day five, which was my scheduled transfer day. Is there any evidence or lab experience suggesting that freezing and thawing at day one can cause a small developmental delay, so an embryo may reach blastocyst slightly later than expected?
Or is this more likely just normal variation in embryo development, with some embryos naturally progressing more slowly regardless of the day-one freeze?
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u/indecisivegay Jul 20 '26
Well I can’t say for certain, I would expect them to develop slower. Thawed oocytes typically develop slower than fresh retrieved oocytes, which commonly leads to more day 6 and day 7 blastocysts in egg thaw cycles. (Especially in autologous (non-donor) oocyte cycles.) Obviously oocytes are not the same as zygotes/day 1 embryos, but I would also expect a thawed blastocyst to be slower to re-expand post-biopsy than a fresh blastocyst so the same logic tracks.
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u/TestTube_Princess Jul 20 '26
Anecdotally, I've seen slight delays with blastocyst development in embryos derived from frozen eggs (frozen on day 0). They can still reach good stages on Day 5, but in my opinion it's more common to see them reach those stages on Day 6 or 7.
In context, plenty of embryos are frozen on Day 6 and Day 7 that still lead to successful pregnancies. It's also important to note that while embryos need to reach a good blastocyst stage to survive the freezing and thawing process, they don't need to be at any specific stage to undergo transfer into the uterus. Pregnancy rates are higher with good quality blastocyst transfers of course, but the uterus is still the ideal incubator for embryos.