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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREEClear out junk files and repair common Windows errorsFree Scan →When more than one sperm fertilizes an egg, the event is called polyspermy. Mammalian eggs normally activate defenses after the first sperm fuses with the egg, limiting further sperm entry. If extra sperm do enter, the resulting additional paternal chromosomes can disrupt development—but not every case of triploidy, an embryo with three chromosome sets, is caused by two sperm.
What polyspermy is—and why it matters
Polyspermy is fertilization of an egg by more than one sperm. Each sperm carries a paternal set of chromosomes. If multiple sperm contribute their chromosomes, the embryo may have more than the usual two sets, a condition called polyploidy. Such an abnormal chromosome complement generally prevents normal development.
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One possible result is triploidy, in which there are three chromosome sets rather than the usual two. But triploidy does not prove that two sperm fertilized the egg: it can also arise if a diploid sperm contributes an extra set or if the egg retains an extra chromosome set.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsHow a mammalian egg limits additional sperm
Fusion with the first sperm activates the egg. One important response is calcium-dependent release of cortical granules, small structures just beneath the egg’s membrane. Their contents alter the zona pellucida, the extracellular coat surrounding the egg, making it less receptive to further sperm binding or penetration.
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In mice, the enzyme ovastacin released by cortical granules cleaves the zona protein ZP2. A 2024 study describes ZP2 cleavage as an evolutionarily conserved component of the permanent egg-coat block to polyspermy. The details of the block, including its timing and the precise contribution of different mechanisms, should not be assumed to be identical in every species.
Two locations, with species-specific timing
| Defense | Where it acts | What is established |
|---|---|---|
| Egg-coat block | Zona pellucida, outside the egg membrane | Cortical-granule release modifies the coat; in mice, ovastacin-mediated ZP2 cleavage is part of the permanent block. |
| Membrane-level block | Egg membrane | Membrane changes also contribute in mammals, but the underlying mechanism is not fully explained. |
A 2020 review notes that mammalian membrane and zona-pellucida blocks are established at approximately the same time. This differs from the more clearly separated fast and permanent blocks often described in non-mammalian animals. The broad purpose is to limit additional sperm, but the mechanisms and timing vary across species.
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What human studies can—and cannot—tell us
Historical figures sometimes cited in discussions of polyspermy are not current estimates of population risk. Robert D. Martin’s 2017 article relays results from older studies:
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- Jacobs and colleagues’ 1978 study found a paternal extra chromosome set in 17 of 21 triploid miscarriage cases with identified parental origin; three had an extra maternal set, and one was unresolved. The authors calculated that about two-thirds of triploid cases were attributable to dispermy—fertilization by two sperm.
- The same 1978 paper estimated triploidy in 1%–3% of detectable conceptions. This is a historical estimate, not a present-day prevalence figure.
- Shi and Martin’s 2000 study reported a mean diploid-sperm frequency below 0.4% in samples of more than 200,000 sperm from each of 10 healthy Chinese men and a similar sample from each of 10 healthy Canadian men. Those small donor groups do not establish a rate for all men.
A 1957 observational comparison reported higher average sperm concentration and motility among men whose partners had repeated miscarriages than among men whose partners had several live births. That comparison does not establish that high sperm concentration causes miscarriage and should not be used as clinical guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why fertilization is not simply a sperm race
The image of many sperm competing in a straight race to an egg can mislead. Michael Bedford cautioned against treating fertilization as a race determined by sperm velocity. The quotation is relayed by Martin from Bedford’s 2008 paper, “Puzzles of mammalian fertilization — and beyond.” Fertilization involves biological interactions and egg defenses, not just which sperm swims fastest.
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