Fertilization isn't determined solely by the fastest or most numerous sperm. Chemical signals in the fluid surrounding the egg can attract some men's sperm more strongly than others.
This attraction depends on the specific pairing of a woman's follicular fluid and a man's sperm, not simply on whether he is her partner.
Separate research suggests that conscious attraction and sperm-egg compatibility may follow different biological mechanisms, meaning they do not always align.
Scientists are still investigating whether these interactions could help explain some cases of unexplained infertility, but the research is still in its early stages.
No clinical test currently exists to measure sperm-egg compatibility, and these findings are not used in fertility diagnosis or treatment.
For decades, the textbook image of fertilization has looked like a race. Millions of sperm released, one egg waiting, and the fastest, strongest swimmer crosses the finish line first.
Recent research suggests that is not how fertilization actually works. The fluid surrounding a human egg might not treat every sperm cell the same way, and the difference has very little to do with speed.¹
This surrounding fluid, called follicular fluid, naturally bathes the egg within the ovarian follicle before ovulation and was the material researchers analyzed in these experiments rather than the egg cell itself.
Researchers have spent the last few years mapping out what actually shapes the outcome, and it turns out to be far stranger, and far more specific, than a simple race.
An egg doesn't sit passively waiting for sperm to arrive. The fluid surrounding it, called follicular fluid, contains chemoattractants, chemical signals that actively draw sperm toward the egg. Sperm respond to these signals by changing how they swim, orienting toward the fluid and accumulating around it.
In 2020, researchers from Stockholm University, the University of Manchester, and Manchester University NHS Foundation Trust set out to test something nobody had tested directly in humans before: does this chemical pull treat all sperm the same, or does it favor sperm from specific men?¹
The team collected follicular fluid and sperm samples from couples undergoing fertility treatment at St Mary's Hospital, Manchester, across two separate experiments, one testing 16 couples, the other testing 44.¹ In both, sperm were exposed to follicular fluid from different women, and researchers counted how strongly each sample was drawn in.
The result wasn't that some women's fluid is simply stronger, or that some men's sperm simply swim better. It was that the specific pairing of a given woman's fluid and a given man's sperm determined how strong the pull was. The same man's sperm could be drawn strongly to one woman's fluid and barely respond to another's.¹
The difference was striking. In the experiments, follicular fluid attracted nearly ten times as many sperm as a neutral control fluid, averaging 435 sperm compared with 45.¹ Researchers caution, however, that fertilization in the body is much more complex and influenced by many interacting factors.
See also: Los Angeles Host the 1st Live Sperm Race to Spotlight Male Fertility
Given that the pull seems to be so specific, it would be reasonable to assume it favors a woman's actual partner. It doesn't, at least not on average.
The researchers tested this directly, comparing how strongly follicular fluid was drawn to a partner's sperm versus a non-partner's. Across both experiments, there was no significant overall difference.¹ Partner sperm wasn't favored, and neither was outside sperm.
This is worth giving some thought, because some popular coverage of this research has slipped into implying that eggs have a bias toward unfamiliar sperm. That's not what the data shows. There's no general preference either way. What the data shows is that the variable that matters is the specific two people involved.
In other words: compatibility, in this context, isn't something either person has on their own. A man's sperm isn't simply "good" or "bad" in the abstract. It only becomes more or less compatible in relation to one specific woman's follicular fluid.¹
If compatibility lives between two people rather than inside either one, a natural question follows: does that extend to attraction itself? A more recent study set out to answer exactly this.
Ten women rated the body odor of eleven men for attractiveness, blind to whose sample was whose.2 The same women's follicular fluid was then tested against the same men's sperm. Everyone was genetically typed for HLA genes, the immune-system gene family better known as MHC, which has a well-documented role in body-odor preference.
The two findings pointed in different directions. Consciously, women rated the odor of men with more similar immune genes as more pleasant.2 At the cellular level, sperm motility was better in pairings with more different immune genes, though only at one measurement point, 60 minutes after exposure.2
It's tempting to read this as the body working against a relationship at a cellular level. It isn't. Conscious attraction and cellular fertilization were never running on the same logic. Scent-based preference evolved to solve a pre-mating problem: who to pursue, often via cues below conscious awareness. Sperm performance evolved to solve a separate, later problem, on a separate timeline, with separate machinery. A disagreement between the two isn't a contradiction, it's two systems, neither one ever under conscious control, doing two different jobs.
Worth keeping the scope in view, too: one gene family, one time point, ten women, eleven men. That narrowness is what makes it credible rather than a tabloid headline, a precise finding, not a verdict on anyone's relationship.
Not as of now. Even within the immune-gene research, results vary depending on which body fluid is tested. A 2021 follow-up study looking at both follicular fluid and cervical mucus found a pattern that ran partly in contrast with the two 2020 studies it was building on.3 The original 2020 chemoattractant study, for its part, found only a weak and inconsistent link between this chemical signalling and actual outcomes during IVF treatment, fertilization rate, embryo quality, pregnancy, and live birth were not reliably predicted by it.¹
This is normal for an active research area, not a sign the science is shaky. No diagnostic test or clinical application currently exists based on any of this work.
Although these findings are intriguing, the studies involved relatively small numbers of participants and will need to be confirmed in larger studies before they can influence clinical practice.
See also: Egg Donor IVF - Who Needs It, How It Works, and Success Factors
Roughly 15 to 30 percent of couples who seek help for infertility are eventually given a diagnosis of "unexplained infertility", a frustrating non-answer for people who want a reason, not just a label.
Daniel Brison, senior author of the 2020 chemoattractant study and Scientific Director of the Department of Reproductive Medicine at St Mary's Hospital, Manchester, has suggested in Manchester News that gamete-level signalling could eventually shed light on some of these cases: research on the way eggs and sperm interact, he noted, "may eventually help us understand some of the currently 'unexplained' causes of infertility in couples."
This is a promising research direction, not a current answer for anyone navigating unexplained infertility today.
Myth: The fastest sperm always fertilizes the egg.
Fact: Speed matters far less than how a specific man's sperm responds to a specific woman's chemical signals.¹
Myth: Eggs are passive, simply waiting for sperm to arrive.
Fact: Eggs are surrounded by fluid that actively releases chemical signals, drawing some sperm in more than others.
Myth: Eggs are biologically "unfaithful" and prefer sperm from outside a relationship.
Fact: No overall bias toward partners or non-partners was found. Attraction depends on the specific pairing, not relationship status.¹
Myth: A mismatch between attraction and cellular compatibility means something is wrong in a relationship.
Fact: The two systems evolved to solve different problems on different timelines. A disagreement between them is expected, not alarming.2
Myth: Doctors can already test for this kind of compatibility.
Fact: No approved clinical test exists yet. This remains active, evolving research.
Fertilization isn't a race with a single kind of winner. It looks more like a negotiation that depends on exactly which two people are involved, mediated through chemical signals that neither person consciously controls.
Understanding that attraction and biological compatibility can run on separate tracks doesn't take anything away from a relationship. It's a reminder that neither system was ever within anyone's conscious control to begin with, which means there's nothing here to manage, worry about, or read as a verdict on a relationship. It's simply how the underlying biology works, in two specific people, at two different points in a much longer story.
Fitzpatrick, John L., Charlotte Willis, Alessandro Devigili, Amy Young, Michael Carroll, Helen R. Hunter, and Daniel R. Brison. "Chemical Signals from Eggs Facilitate Cryptic Female Choice in Humans." Proceedings of the Royal Society B: Biological Sciences 287, no. 1928 (2020): 20200805. https://pubmed.ncbi.nlm.nih.gov/32517615/
Jokiniemi, Annalaura, Tanja Turunen, Mikko Kohonen, Martina Magris, Jarmo Ritari, Liisa Kuusipalo, Jukka Partanen, and Jukka Kekäläinen. "Female-Mediated Selective Sperm Activation May Remodel Major Histocompatibility Complex-Based Mate Choice Decisions in Humans." Heredity 134, no. 6 (2025): 321-330. https://pubmed.ncbi.nlm.nih.gov/40346315/
Magris, Martina, Annalaura Jokiniemi, Jarmo Ritari, Liisa Kuusipalo, Satu Koskela, Jukka Partanen, and Jukka Kekäläinen. "Structural Dissimilarity of Partners' Immune Genes Increases Sperm Viability in Women's Reproductive Tract." Journal of Evolutionary Biology 34, no. 7 (2021): 1125-1132. https://academic.oup.com/jeb/article/34/7/1125/7326574