Does the Egg Choose the Sperm? What the Research Actually Shows
Compiled and fact-checked from cited sources, reviewed by our editorial team. · Last updated:
The version taught in school went like this: millions of sperm set off in a race, the fastest one reaches the egg first and wins. The most important part of that story is wrong.
The egg is not a finish line. It is a filter. It does not wait passively for whatever arrives: it calls sperm chemically, it does not call all of them equally, and the moment one gets inside it slams the door shut in a matter of seconds.
Twenty years of research have filled in this picture step by step. Below is what is known — and, just as importantly, what it does not mean.
This article reports scientific findings in reproductive biology. It is not medical advice, diagnosis or treatment. If you have a personal question about conception or fertility, speak to a doctor.
Why the race story is wrong
The error is not in the word “race”. It is in the assumption about who sets the rules of the race.
Hundreds of millions of sperm are released. The number that reach the vicinity of the egg is a few hundred to a few thousand. That enormous gap is not the outcome of a speed contest — it is the outcome of the female reproductive tract being a filter from end to end.
Several separate mechanisms guide and cull sperm along the way:
- Rheotaxis: sperm tend to swim against the flow of fluid, which sorts out the ones heading the right way.
- Thermotaxis: the region where the egg sits is slightly warmer than its surroundings, and sperm orient towards that difference.
- Chemotaxis: chemical signals released around the egg pull sperm towards the source. This is where the real story is.
So it is not “the fastest one wins”. It is that a handful of sperm that can turn the right way, read the right signal and survive the filtering make it to the final stage.
How the egg calls
When the egg leaves the ovary it does not travel alone: it is surrounded by a cloud of cumulus cells. One of the main substances those cells secrete is progesterone.
As the cumulus cells release progesterone, a concentration gradient forms around the egg — like a scent trail that gets stronger closer to its source. Sperm detect that gradient and swim towards the increasing concentration. The internal signalling pathways that make this work have been mapped: a cyclic-AMP cascade fires first, followed by calcium mobilisation.
The crucial point is this: the signal is produced by the egg’s side. Sperm are not wandering at random and happening to collide with an egg. They are going where they are called.
The real question: is the call the same for everyone
This is where a study published in 2020 changed the picture.
Researchers from Manchester Metropolitan University and Stockholm University, publishing in Proceedings of the Royal Society B, compared follicular fluid from women — the fluid the egg develops in, which carries chemoattractants — against sperm from different men.
The finding was that follicular fluid from different women consistently attracts sperm from different men to different degrees. One woman’s follicular fluid pulled one man’s sperm more strongly; another woman’s pulled a different man’s. This was not random scatter but a repeatable pattern.
The part that drew the most attention was the second finding: that chemical preference did not line up with the woman’s choice of partner. The sperm most strongly attracted by a woman’s follicular fluid was not necessarily her partner’s.
The researchers framed this as cryptic female choice — selectivity that operates after mating, at the level of the gametes, independently of conscious preference.
What this finding does not mean
This section matters as much as the finding itself, because this is where headlines distort it most.
It does not mean “the egg picks the best baby.” The study showed a difference in attraction strength. It did not show whether that difference translates into fertilisation success, a pregnancy continuing, or the health of a child.
It does not mean you are “incompatible” with your partner. Follicular fluid attracting another man’s sperm more strongly does not mean a couple cannot conceive. Billions of people exist not in spite of this mechanism but alongside it.
It was done in the laboratory. The experiment brought fluid and sperm together under controlled conditions. How much of the effect survives inside the complex environment of the body is a separate question.
It is not a test or a screening tool. There is no clinical procedure that measures “follicular fluid compatibility” and guides couples. This is a basic-science finding.
After one sperm enters: the door closes
The egg’s selectivity does not end with the call. The decisive act happens at the moment of contact.
The egg is surrounded by a glycoprotein coat called the zona pellucida. A sperm has to get through it, and passage is not free: it must be able to bind to specific proteins in that coat.
The moment one sperm gets in, the egg responds within seconds and the coat hardens. At least three separate mechanisms have been described:
- An enzyme called ovastacin cleaves the ZP2 protein in the coat, destroying the region sperm bind to — the lock is dismantled.
- Zinc released from the egg’s cortical granules (the “zinc spark”) disrupts the motility of approaching sperm.
- Transglutaminase-2 cross-links ZP3 proteins, physically stiffening the coat.
This is the block to polyspermy, and it has a vital function: an egg entered by more than one sperm carries too many chromosomes and cannot develop. So the egg “closing the door” is not a matter of preference — it is the condition under which an embryo can exist at all.
What happens on the sperm’s side
Selectivity is not one-directional. Sperm also have to complete a maturation process before they can fertilise anything: capacitation. During their time in the female tract the sperm’s membrane changes and its swimming pattern shifts, and only those that complete this change become capable of fertilising the egg.
So both sides do their own filtering: the sperm that cannot make the journey are removed, and so are the ones that cannot open the door.
What it means for fertility and IVF
This research has not translated into a treatment. But it offers a frame for why some couples are told they have “unexplained infertility”: even in a sample where sperm count and motility look normal, compatibility at the level of the gametes may be a factor.
Intracytoplasmic sperm injection (ICSI) has a particular place in this picture. In that technique the sperm is placed directly inside the egg, so both the journey and the coat are bypassed — which means both layers of natural selectivity are switched off. That is not a flaw in the method; it is simply one of the reasons it is used only where it is needed.
Frequently asked questions
Does the egg choose the sperm? Not in the everyday sense of choosing, but it is not passive either. It attracts sperm with chemical signals, and that attraction is not equally strong for all men. It also actively prevents other sperm from entering once one is in.
Does the fastest sperm win? No. Speed alone does not decide it. Orientation, reading the chemical signal and surviving the filtering of the female tract matter more.
How many sperm reach the egg? Of the hundreds of millions released, only a few hundred to a few thousand reach the region where the egg is.
How does the egg attract sperm? The cumulus cells surrounding the egg secrete progesterone, which forms a concentration gradient. Sperm detect it and swim towards the stronger concentration.
What exactly did the follicular fluid study show? Published in Proceedings of the Royal Society B in 2020, it showed that follicular fluid from different women consistently attracts sperm from different men to different degrees, and that this preference did not align with the woman’s partner choice.
Does that mean I am incompatible with my partner? No. The study measured attraction strength, not conception success or couple compatibility. It produces no clinical test or recommendation.
Why can only one sperm enter? An egg entered by more than one sperm has too many chromosomes and cannot develop. So as soon as the first sperm is in, the egg hardens its outer coat and shuts the others out.
What is the zona pellucida? The glycoprotein coat around the egg. A sperm must bind to and cross it to fertilise; immediately after fertilisation it hardens and closes the door.
How does the block to polyspermy work? Three mechanisms together: ovastacin cleaves the ZP2 sperm-binding region, zinc released from cortical granules disrupts approaching sperm, and transglutaminase-2 cross-links ZP3 to stiffen the coat.
What is capacitation? The maturation sperm undergo in the female reproductive tract that makes them capable of fertilising. Sperm that do not complete it cannot fertilise the egg.
Is any of this used in IVF? Not as a test. But in ICSI the sperm is injected directly into the egg, bypassing the natural selection layers described here.
Are twins an exception? No. Fraternal twins come from two eggs fertilised by two sperm; identical twins come from one fertilised egg that later splits. In both cases the one-egg-one-sperm rule holds.
Does the egg choose the baby’s sex? No. Sex is determined by whether the fertilising sperm carries an X or a Y chromosome. There is no evidence the egg makes that choice.
Sources
- Fitzpatrick JL et al. — Chemical signals from eggs facilitate cryptic female choice in humans. Proceedings of the Royal Society B, 2020. https://royalsocietypublishing.org/rspb/article/287/1928/20200805/85674/Chemical-signals-from-eggs-facilitate-cryptic
- Full text of the same study (PMC archive). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7341926/
- Teves ME et al. — Molecular mechanism for human sperm chemotaxis mediated by progesterone. PLOS ONE. https://journals.plos.org/plosone/article?id=10.1371%2Fjournal.pone.0008211
- Que EL et al. — Zinc sparks induce physiochemical changes in the egg zona pellucida that prevent polyspermy. https://pmc.ncbi.nlm.nih.gov/articles/PMC5439353/
