“Immature Eggs Mean It’s Over?”… Cultivating Them One More Day Led to Normal Embryos and Even a Healthy Baby
GV Oocytes Harvested on Retrieval Day Cultivated for an Additional 24 Hours… Roughly Half Revive as Mature Eggs While Blastocyst Progression Power Is Reduced, Chromosomal Normality Rates Among Surviving Blastocysts Remain Competitive Rescue Oocytes Open Transfer Opportunities for Patients Lacking Normal Embryos; Delivering Mothers Increased from 15 to 18
On the day of egg retrieval in In Vitro Fertilization (IVF), a patient’s emotions sway with every single number.
You feel a wave of relief hearing, “We retrieved 8 eggs,” only to have joy instantly replaced by anxiety when the doctor explains, “Four are mature, and the rest are immature.” This is because eggs utilized for fertilization are traditionally expected to be fully mature MII-stage oocytes.
Are eggs that remain unripened at the time of retrieval truly finished for good?
Recent research has delivered an intriguing answer: “Let’s wait just one more day.”
Researchers in Spain (IVI Valencia) and Italy chose not to discard the immature eggs of women who yielded fewer mature eggs than anticipated during IVF cycles, instead culturing them for up to 24 additional hours.
As a result, some of those eggs matured into viable oocytes, underwent successful fertilization and blastocyst development, yielded euploid (normative chromosome) embryos, and ultimately resulted in live births. The study was published in the international journal Human Reproduction on July 13, 2026.
The Potential of ‘GV Oocytes’
The research team focused on GV oocytes. GV stands for ‘germinal vesicle.’ While it sounds complex, it simply refers to the youngest stage of an oocyte, which is not yet prepared for fertilization.
In standard IVF, clinicians select and fertilize oocytes matured up to the MII stage, frequently setting aside immature eggs stuck at the GV or MI stages due to a presumed lower developmental competence. According to the researchers, roughly 10% of retrieved eggs are typically observed at the GV stage.
The study cohort was deliberately restricted. The researchers analyzed the treatment outcomes over an 8-year period from 2017 to 2024 for 47 carefully selected women under the age of 39 who underwent PGT-A, where fewer than six MII mature eggs were retrieved, maturity rates fell short of expectations, and at least four GV oocytes were recovered.
A total of 266 GV oocytes were gathered from these patients. The research team placed these eggs into a time-lapse incubator to wait for up to an additional 24 hours.
And the oocytes began to stir.
Out of the 266 total eggs, 127 matured to the MII stage within 24 hours.
This represented 47.7% of all eggs, with a per-patient average maturation rate of 53.7%. Through this ‘rescue maturation,’ the average number of mature eggs available per patient increased from 4.4 to 7.1. The researchers calculated a 75% average relative increase in MII egg counts per patient.
Put simply, for a patient who initially thought, “I only have 4 mature eggs” based solely on retrieval day, overnight cultivation yielded an additional 2 to 3 mature eggs.
Development and Chromosomal Health
Naturally, late-maturing eggs were not identical in every aspect to eggs that ripened perfectly right from the start.
The fertilization rate of normally retrieved MII eggs stood at 71.4%, while eggs matured after an extra day of culture achieved 66.1%, showing no statistically stark divergence.
However, a noticeable gap emerged in their capacity to survive past fertilization to the blastocyst stage. The blastocyst formation rate for normal MII eggs was 49.0%, whereas rescue-matured oocytes lagged at 20.2%. In other words, while the late-ripening eggs fertilized quite well, a significant portion dropped out during the journey toward the blastocyst stage.
Yet, this is where the study’s most fascinating finding emerged.
For the oocytes that managed to defy the odds and survive to the blastocyst stage, the narrative shifted.
When subjected to PGT-A screening, among 72 blastocysts generated from normally matured eggs, 51.4% (37 embryos) were euploid. Conversely, among 17 blastocysts created from rescue-matured oocytes, 76.5% (13 embryos) possessed normal chromosomes.
While the sample size was too small to establish definitive statistical superiority across the entire cohort, it firmly dispelled the assumption that “embryos made from late-maturing eggs must possess inferior chromosomes.”
Translating Lab Success into Real-World Babies
What mattered more than statistics was the single, tangible opportunity afforded to actual patients.
Thirteen patients secured euploid embryos from rescue-matured oocytes. Across four treatment cycles, no normal embryos emerged from natively mature eggs, yet rescue GV oocytes successfully produced normal embryos.
For these individuals, if the GV oocytes had been discarded, their transfer-eligible embryo count for that cycle would have read ‘zero.’ Factoring in rescue oocytes expanded the number of patients securing one or more euploid embryos from 26 to 30.
And at last, babies were born.
Ten transfers utilizing euploid embryos derived from rescue-matured oocytes culminated in 4 live births. Among the total patient cohort, women achieving at least one successful live birth rose from 15 (excluding rescue oocytes) to 18 (including them).
The researchers calculated this as a 20% relative increase in cumulative live birth patient counts. Gestational age and birth weight of the resulting infants showed no distinct differences compared to embryos originating from conventional MII eggs. However, the absolute number of birth cases remains too small to draw definitive conclusions regarding safety.
Conclusion and Clinical Perspective
This study does not imply that “all immature eggs should henceforth be cultured for one more day.”
The research cohort was limited to 47 participants and represented a retrospective study conducted at a single private fertility center.
Furthermore, it focused exclusively on a specialized subset of patients under 39 who unexpectedly yielded few mature eggs. The researchers emphasize that rescue IVM should not be applied routinely to every IVF patient, but rather interpreted as an auxiliary strategy worth considering for low-prognosis patients who yield very few mature eggs alongside multiple immature GV oocytes.
Even so, the message conveyed to fertility patients is profound.
Anyone who has undergone egg retrieval understands the weight of a ‘single egg.’ One egg can become one embryo, one embryo a blastocyst, and that blastocyst a euploid embryo. Particularly for women with low retrieval yields, one or two immature eggs are never mere statistical digits in a laboratory log.
Up to this point, certain eggs were barred from even entering the IVF starting line simply because they lacked full ripeness at the exact moment of aspiration. This study granted those eggs 24 more hours of time.
Not every egg survived. Their capacity to reach the blastocyst stage was undeniably lower. Yet a portion of them matured, fertilized, formed blastocysts, and turned into normal, healthy chromosomes.
And some of those eventually became babies.
The verdict of “immature” on the day of egg retrieval may not spell the final sentence for every single oocyte.
※ This article was synthesized to aid general reader understanding based on clinical experiences in obstetrics, gynecology, and reproductive medicine alongside recent research data. It does not replace a specific individual’s diagnosis or treatment, and actual medical judgment must be made through consultation with a specialist.
※ Images: Created using generative AI (ChatGPT, OpenAI) to provide visual references supporting understanding.