Polyspermy, when more than one sperm enters an egg during fertilization, is bad because it gives the resulting embryo too many chromosomes and too many cellular structures to divide properly. In humans, this almost always leads to the embryo’s death, either through early miscarriage or failure to develop past the first few cell divisions. It is one of the most common causes of pregnancy loss, detected in roughly 10% of spontaneously aborted human pregnancies.
Too Many Chromosomes, Too Many Problems
A normal human embryo gets 23 chromosomes from the egg and 23 from a single sperm, totaling 46. When two sperm fertilize the same egg, the embryo ends up with 69 chromosomes, a condition called triploidy. That extra set of genetic instructions throws off nearly every aspect of development. Cells can’t properly regulate which genes to turn on or off, and the balance of proteins the embryo needs to grow is disrupted from the very start.
The majority of triploid human embryos result from two sperm fertilizing one egg. These pregnancies rarely make it far. The estimated prevalence of triploidy at 16 to 20 weeks of pregnancy is about 1 in 5,000, and among live births it drops to roughly 1 in 10,000. The few infants born with complete triploidy typically survive only hours or days.
How Extra Sperm Wreck Cell Division
The chromosome problem is only part of the story. Each sperm brings a structure called a centrosome into the egg. Centrosomes organize the machinery that pulls chromosomes apart when a cell divides. In normal fertilization, the single sperm’s centrosome pairs with material already in the egg to form a two-poled spindle, neatly splitting chromosomes into two daughter cells.
When extra sperm enter, their additional centrosomes create a multipolar spindle, something like a tug-of-war with three or four teams instead of two. Chromosomes get yanked in multiple directions at once. Some daughter cells end up with a normal set of chromosomes, others get only a partial set from one of the extra sperm, and some cells get random, jumbled combinations. This produces what’s called genomic mosaicism: a patchwork embryo where different cells have different chromosome counts. Development stalls almost immediately, typically at the first or second cell division.
What Happens in a Triploid Pregnancy
Most triploid embryos never implant or are lost so early that the pregnancy goes unrecognized. Those that do implant far enough to be detected on ultrasound show distinctive abnormalities depending on whether the extra chromosomes came from an additional sperm (paternal origin) or from the egg itself failing to shed a set of chromosomes (maternal origin). Since the reader’s question is about polyspermy, paternal origin is the relevant scenario.
Pregnancies where the extra chromosome set comes from the father tend to show an enlarged placenta with cystic, grape-like changes. This is a partial molar pregnancy, which occurs in 60% to 80% of these cases. The fetus is typically mildly growth-restricted, sometimes with a disproportionately small head. These pregnancies require medical management because molar tissue can, in rare cases, persist and cause complications after the pregnancy ends.
How Eggs Normally Block Polyspermy
Given how catastrophic polyspermy is, eggs have evolved two overlapping defense systems that kick in the moment the first sperm makes contact.
The fast block happens within seconds. The egg’s outer membrane rapidly shifts its electrical charge, essentially flipping a switch that repels other sperm trying to fuse with it. This electrical change persists for at least a minute, buying time for the second, more permanent defense to activate.
The slow block takes tens of seconds to a few minutes. Tiny packets just beneath the egg’s surface, called cortical granules, burst open and release enzymes into the protective shell surrounding the egg (the zona pellucida in mammals). These enzymes chemically harden the shell and destroy the specific proteins that sperm use to latch on. Once this process is complete, no additional sperm can physically penetrate the egg. The two systems overlap so there’s no gap in protection.
Why Polyspermy Still Happens in IVF
During standard IVF, eggs are placed in a dish with thousands of sperm, and the natural blocking mechanisms sometimes can’t keep up. The incidence of polyspermy during conventional IVF is 2% to 10%. This is one reason embryologists check fertilized eggs under a microscope the morning after insemination, looking for the telltale sign of three pronuclei (the small circular structures that form around each set of chromosomes) instead of the normal two. Embryos with three pronuclei are identified and not transferred.
A technique called ICSI, where a single sperm is injected directly into the egg, largely sidesteps this problem by removing the competition entirely. It’s now the most widely used fertilization method in many IVF clinics, partly for this reason.
Some Animals Handle Polyspermy Just Fine
Not every species treats polyspermy as a disaster. Birds, some reptiles, and certain salamanders use what biologists call physiological polyspermy, where multiple sperm routinely enter the egg as a normal part of fertilization. In birds, several sperm typically penetrate the egg’s germinal disc, but only one fuses its genetic material with the egg’s nucleus. The extra sperm are simply broken down and absorbed without contributing chromosomes to the embryo.
Birds actually need this to happen. Their eggs are so large that ensuring at least one sperm reaches the fertilization site requires multiple sperm to enter. The key difference is that these species have evolved mechanisms to let extra sperm in physically while still keeping only one set of paternal DNA in the final embryo. Mammals never evolved this workaround, which is why the electrical and chemical blocking systems are so critical in human reproduction.