A 2025 case report describes a live birth from a frozen immature egg matured through rescue IVM, suggesting that immature eggs in freezing cycles may have reproductive potential and should not be routinely discarded.
After eggs are collected for freezing, not every egg is mature. A significant fraction can be immature, often in the metaphase I (MI) stage, and are usually discarded.
Laboratory techniques like rescue in vitro maturation (IVM) can push some MI oocytes to progress to maturity to the metaphase II (MII) stage.
Studies have shown that while embryos from rescue IVM may develop less efficiently than those from naturally matured eggs, once a good-quality blastocyst forms, outcomes such as euploidy and live birth rates can be similar. This data is reviewed below.
But there is little evidence on whether MI oocytes can be used for egg freezing. A new case report by Niu et al. (2025) offers a unique real-world example of a live birth from an embryo derived from a frozen MI oocyte.
Want to learn more about egg maturity and development? Check my Complete guide to egg quality.
🔗 Original studies are referenced in this post or within the linked Remembryo posts.
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Summary of case report
The report describes a 36-year-old woman who froze 28 eggs in 2021. Two years later, after marrying, she returned to use them.
Twelve eggs were thawed: four were mature (MII) and eight were MI. Five of the MI oocytes successfully matured to MII after about eight hours in culture, and these were inseminated by ICSI along with the originally mature oocytes.
Ultimately, two blastocysts developed, both originating from the initially immature oocytes. One of these, graded 3AA, was transferred and resulted in the birth of a healthy baby boy.
Conclusions
While MI immature eggs are usually discarded, this case report shows that rescue IVM can lead to mature oocytes capable of forming blastocysts and healthy live births.
Surprisingly, none of the embryos from the thawed mature oocytes developed to blastocyst stage, but the immature eggs that underwent rescue IVM produced two blastocysts, including the one that led to a live birth.
A recent study showed that about 1 in 12 births (8.7%) would not have happened without rescue IVM in cycles involving poor responders, with additional frozen embryos from the technique that could potentially lead to more live births in the future (read more in my post About 1 in 12 poor prognosis patients have live birth thanks to rescue IVM).
A lot of immature eggs are routinely discarded — A study of nearly 1500 women freezing their eggs, including almost 20,000 eggs, showed that 24% of these were discarded because they were immature (Cobo et al. 2016). The results of this case report suggest that these eggs may have reproductive potential and “the routine practice of MI oocytes should be reconsidered,” the authors write.
Limitations: This is a single case report, so its findings cannot be generalized. Rescue IVM itself is still under-studied, and although healthy live births have been reported, larger studies are needed to confirm its safety and clinical value.
Want to read more about rescue IVM and its success rates?
Researchers in a 2022 study compared outcomes from mature M2 eggs and M1-M2 eggs that underwent “rescue in vitro maturation” and found that M2 eggs had improved fertilization, blastocyst conversion and euploidy rates, with no difference in pregnancy or live birth rates. Read more.
Researchers in a 2023 study used a procedure called "rescue in vitro maturation" to culture immature eggs collected after retrieval overnight, and found that a high proportion became mature and had comparable fertilization to sibling mature eggs, with many going on to form euploids. Read more.
Researchers in a 2023 study used rescue IVM and found that many immature eggs became mature and had comparable fertilization and euploid rates to sibling mature eggs, although there was a decrease in blastocyst formation rates. Read more.
Reference
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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.
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