Does the Embryologist Change IVF Success? What 6,904 Frozen Transfers Reveal

Does the embryologist who freezes or warms an embryo affect IVF pregnancy outcomes? A study of 6,904 single frozen blastocyst transfers found no significant clinical pregnancy differences according to the vitrifying embryologist. A possible signal for the warming operator did not reach statistical significance. The finding concerns experienced staff working within standardized, quality-controlled laboratories—not a claim that embryologists are unimportant.

Freezing and warming embryos are now routine parts of IVF. Widespread use of ultra-rapid blastocyst vitrification has helped expand frozen embryo transfer. Yet even with the same equipment and protocols, could the person performing the procedure make a difference?

Researchers from IRCCS Humanitas Research Hospital, Humanitas University and collaborating institutions in Italy examined clinical data to address that question. The study appeared in the October 2026 issue of Fertility and Sterility.

Freezing and warming still involve skilled hands

Vitrification is more than putting an embryo in a freezer. Embryos must be exposed to cryoprotectant solutions and handled rapidly before storage in liquid nitrogen. Warming also involves prescribed solutions, timing and transfer steps.

Even when reagents, equipment and written instructions are the same, skill, speed and precision can influence how a procedure is performed. This human factor was the focus of the study.

The researchers analyzed 6,904 single frozen blastocyst transfers performed at one center between January 2019 and June 2023. Eleven embryologists performed vitrification and 12 performed warming.

These were not beginners. Each included embryologist had already performed at least 500 freezing/warming procedures before 2019 and underwent regular quality control and performance assessment.

No significant pregnancy difference by vitrifying embryologist

Who vitrified the blastocyst was not significantly associated with clinical pregnancy. The result remained after adjustment for female age, blastocyst grade, treatment year, preimplantation genetic testing and the day the blastocyst developed.

For warming operators, there was a somewhat stronger suggestion of between-embryologist variation, but it did not meet the threshold for statistical significance. The number of procedures an embryologist had previously performed also showed no significant association with clinical pregnancy.

The team then performed external confirmation using 2,767 cycles from another large fertility center. That analysis likewise found no significant evidence of clinical pregnancy differences according to the person who vitrified or warmed the embryo.

This is not a reason to dismiss embryologists—or hunt for a famous one

The findings should not be read as saying embryologists do not matter to IVF. The context points in the opposite direction: the participants were experienced professionals following standardized procedures in large centers with ongoing quality control.

One interpretation is that proper training, shared protocols and continuous review can reduce variation attributable to who performs freezing or warming.

For patients, the study does not suggest that finding one particular pair of skilled hands is the key. It highlights the laboratory's approach to standardization, staff training, competence assessment, embryo survival monitoring and clinical quality control.

IVF outcomes also depend on the laboratory system

Patients usually see the fertility specialist, but embryologists contribute throughout the period from egg retrieval to fertilization, culture, embryo observation, freezing and warming. The study quantified one part of that work: potential operator variation during cryopreservation procedures.

There are limitations. It was a retrospective analysis, and the person who vitrified an embryo could differ from the person who later warmed it, making their individual contributions difficult to separate completely. The researchers called for prospective research.

The message is therefore about developing skilled staff, standardizing procedures and continually reviewing outcomes, rather than ranking whose hands are best. Freezing and warming may be brief, but they are important steps. A laboratory system that enables every embryologist to work to consistent methods and standards deserves attention.

Study source

This article is based on “Association between embryologist-related variability in blastocyst vitrification/warming and reproductive outcomes: a single-center retrospective study with external confirmation analysis,” by Federico Cirillo and colleagues at IRCCS Humanitas Research Hospital, Humanitas University and other institutions. It appeared in Fertility and Sterility, volume 126, issue 4, October 2026. PMID: 42208726. DOI: 10.1016/j.fertnstert.2026.05.157.