“The Liquid Where Embryos Spend Five Days: How Much Do We Really Know?… The Secrets of IVF ‘Culture Medium’”
Examining Commercial Culture Media: Formulations Differ by Product… Over 700 Unlisted Human Proteins Detected in Unused Media Protein Compositions Vary Even Within the Same Product Depending on Manufacturing Lots… ‘Albumin’ Emerges as the Key Variable 2026 Randomized Controlled Trial Shows Cumulative Live Birth Rates of 41.0% vs. 34.5% Depending on Culture Medium… “Media Is a Crucial Variable in IVF”
Patients undergoing In Vitro Fertilization (IVF) ride an emotional rollercoaster over the number of retrieved eggs. They check fertilization rates, ask whether embryos reached Day 3 or Day 5, and scrutinize embryo grades and endometrial thickness.
Yet, hardly anyone asks what fluid that embryo was immersed in during those crucial initial days: the culture medium.
After fertilization, the embryo does not immediately enter the maternal uterus. It spends several days inside an incubator in the IVF laboratory, submerged in a small culture dish. This culture medium contains glucose, lactate, amino acids, electrolytes, and proteins. For an embryo, it serves as food, water, and its very first physical environment.
Recent studies reveal that this “water” is far more intricate and variable than previously assumed.
Over 700 Human Proteins Detected in Fresh, Unused Medium
A 2025 study published in the Journal of Assisted Reproduction and Genetics (JARG) highlighted this complexity.
Researchers collected 85 samples across 13 manufacturing lots of commercial single-step culture media from two manufacturers, analyzing both pre-use and post-use samples via mass spectrometry-based proteomics. Strikingly, over 700 human proteins not listed on product labels were detected in brand-new, unused culture media that had never housed an embryo.
Even more notable was that the manufacturing lot of the culture medium differentiated protein profiles more distinctly than the developmental outcomes of the embryos themselves. In other words, even within the exact same brand and product, the protein composition varied depending on the production batch. Differences were observed not only between separate manufacturers but also between lots from the exact same manufacturer.
Where did these unlisted proteins originate? The researchers pointed to Human Serum Albumin (HSA).
Albumin is a critical protein component in IVF culture media. When researchers compared protein-free media supplemented with either blood-derived purified albumin or recombinant human albumin, the media containing serum-derived albumin revealed numerous co-purified companion proteins, whereas the recombinant albumin medium showed none. Furthermore, the types and quantities of these co-purified proteins varied across different production lots of serum-derived albumin.
This finding does not imply that “700 hazardous substances were detected.” The key takeaway is not proof of toxicity, but the demonstration that an unlisted background protein profile exists and that its composition lacks consistency. For research aiming to identify embryo-secreted biomarkers in spent culture media to predict embryo viability, this uncharacterized background represents a major confounding variable.
Same Product Label, Disparate Formulations
Another study demonstrated significant variation across media brands.
A 2025 study published in Human Reproduction directly measured the concentrations of 40 individual components across 80 samples of commercial human embryo culture media and protein supplements. Comparing glucose, lactate, pyruvate, various amino acids, and electrolytes yielded a straightforward conclusion: no two commercial media were identical.
Concentrations of lactate, glycine, potassium, and other essential nutrients diverged widely across brands, and formulations differed even between distinct brands marketed under the same parent company. The researchers criticized the lack of full disclosure regarding the precise compositions and concentrations in commercial media formulations.
To a patient, culture medium appears as a uniform drop of clear liquid. From the embryo’s biological perspective, however, the metabolic environment—fluctuations in glucose, amino acids, and electrolytes—can differ substantially depending on the chosen medium.
Removing Albumin Compromises Sperm Viability and Antioxidant Defense
Albumin is not merely an inert protein filler.
A study published in Reproductive BioMedicine Online (RBMO) evaluated the antioxidant capacity of various commercial IVF media. Significant differences were observed in how effectively various media protected sperm motility and DNA integrity, with media that best supported sperm function generally exhibiting superior antioxidant capacity.
Albumin was central to this protective mechanism.
When researchers selectively removed albumin from the media, the ability to maintain 24-hour total and progressive sperm motility dropped sharply. The media’s capacity to suppress mitochondrial reactive oxygen species (ROS) and prevent sperm DNA damage was severely compromised.
Re-adding albumin restored these protective functions. The researchers concluded that albumin’s intrinsic antioxidant activity and its ability to bind and neutralize harmful metabolic byproducts play an indispensable protective role.
This presents a noteworthy dynamic: while serum-derived albumin introduces lot-to-lot protein variability, that very same albumin serves as an essential protector against cellular oxidative stress. Albumin is neither a simple additive nor an impurity to be casually eliminated; rather, the focus must be on utilizing highly purified, strictly standardized, and well-characterized albumin sources.
Does Culture Medium Influence Actual Live Birth Rates?
The ultimate clinical question remains: “Does the choice of culture medium alter the probability of delivering a baby?”
A randomized controlled trial conducted across 5 fertility centers in the Netherlands, presented in the July 2026 conference supplement of Human Reproduction, provided notable comparative data.
Analyzing 1,013 IVF patients to compare two commercial media—CSCM and G5—the cumulative live birth rate (encompassing all fresh and frozen transfers derived from a single egg retrieval cycle) was 41.0% in the CSCM group versus 34.5% in the G5 group, a difference of 6.5 percentage points. Fertilization rates (68.2% vs. 65.7%) and usable embryo rates (40.7% vs. 38.2%) also favored the CSCM cohort.
However, premature conclusions must be avoided:
- In adjusted analyses, the p-value for the difference in cumulative live birth rates was p = 0.057, narrowly missing the conventional statistical significance threshold of 0.05.
- Total enrollment (1,016 randomized / 1,013 analyzed) fell short of the originally calculated target of 1,980 participants.
While this single trial cannot definitively establish that one commercial medium is universally superior, a 6.5 percentage point difference underscores that embryo culture media cannot be regarded as inert, interchangeable laboratory consumables.
From “Which Medium?” to “How Strictly Is It Controlled?”
Modern IVF laboratories meticulously control incubator temperature, oxygen tension, culture duration, and morphokinetic imaging. However, accumulating research indicates that the very culture media in direct contact with developing embryos often lack fully disclosed formulations and exhibit noticeable batch-to-batch variation.
This does not suggest that patients must attempt to dictate specific media brands to their fertility clinics, as evidence remains insufficient to declare one product universally best for all clinical scenarios.
Instead, future clinical priorities must center on:
- Greater formulation transparency from manufacturers
- Stricter batch-to-batch homogeneity during production
- Rigorous laboratory-level lot tracking and quality control
- Long-term clinical registries tracking outcomes across distinct media types
A newly fertilized zygote spends its first few days surviving in a microscopic fluid volume of only tens to hundreds of microliters. Under the microscope, the embryo appears as the sole protagonist while the culture droplet seems like a transparent backdrop. Recent research, however, delivers the opposite conclusion: culture media is not merely background scenery—it is the embryo’s very first living environment.
※ This article was synthesized based on studies published in the Journal of Assisted Reproduction and Genetics (2025), Human Reproduction (2025, 2026), and Reproductive BioMedicine Online (2025). It does not replace individual medical diagnosis or clinical care, and specific medical decisions must be made through consultation with a specialist physician.
※ The images used in this article were generated using generative AI (ChatGPT, OpenAI) as visual references to aid understanding and do not depict real individuals.