A fertilized egg is an egg cell that has been penetrated by a sperm cell, combining genetic material from two parents into a single new cell called a zygote. This applies across species, from the chicken eggs you might eat for breakfast to the human eggs involved in pregnancy and fertility treatments. The term comes up in two very different contexts, and most people searching want to understand one or both: what makes an egg “fertilized” in biological terms, and whether the eggs sold in grocery stores are fertilized.
How Fertilization Works at the Cell Level
Every egg cell (from a chicken or a human) starts out with only half the chromosomes needed to create a new organism. A human egg carries 23 chromosomes; a sperm carries the other 23. When a sperm successfully enters the egg, those two half-sets combine into a full set of 46 chromosomes, one copy of each pair from each parent. That single merged cell is now a zygote, and it contains the complete genetic blueprint for a new individual.
The moment the first sperm fuses with the egg, the egg locks out all other sperm through a rapid two-part defense. First, the egg’s outer membrane changes almost instantly, shedding the receptor protein that sperm use to latch on. Second, tiny packets called cortical granules release enzymes that harden the egg’s protective outer shell, making it physically impenetrable. This prevents a dangerous situation called polyspermy, where multiple sperm entering the egg would create a nonviable cell with too many chromosomes.
What Happens After Fertilization
Once fertilized, the zygote begins dividing. In humans, the timeline follows a predictable pattern. By day one, the single cell has split into two cells. By day two, there are four. By day three, roughly twelve. By day four, the cluster reaches 16 to 32 cells and is called a morula, a solid ball still enclosed in the egg’s original outer shell.
Around day five, the embryo has grown to 50 to 150 cells and transitions into what’s called a blastocyst. At this point it begins breaking out of its shell. By the second week, the blastocyst embeds itself into the lining of the uterus, a process called implantation. This entire journey, from fertilization to implantation, takes place over roughly 10 to 14 days.
In a chicken egg, the process is similar in principle but plays out differently. Fertilization happens internally before the shell even forms. By the time a fertilized chicken egg is laid, cell division has already been underway for about 24 hours. If the egg is kept warm (incubated), development continues toward a chick. If it’s collected and refrigerated, development stops.
How to Tell if a Chicken Egg Is Fertilized
You can identify a fertilized chicken egg by looking at the small white spot on the yolk, called the germinal disc. In an unfertilized egg, this spot appears as a small, solid white dot. In a fertilized egg, the spot is noticeably larger and looks like a ring or bullseye, with a somewhat clear center. That clear center indicates that cell division has already begun, with both male and female cells present.
This difference is only visible when you crack the egg open and inspect the yolk closely. From the outside, fertilized and unfertilized eggs look identical.
Are Grocery Store Eggs Fertilized?
Nearly all eggs sold in grocery stores are unfertilized. Commercial egg-laying operations keep hens without roosters, so the eggs hens produce never encounter sperm. These eggs are the hen’s reproductive cells, released on a regular cycle regardless of whether a rooster is present, similar to how human ovulation occurs without fertilization.
On the rare occasion a fertilized egg does enter the supply chain (more common with small farms or free-range operations where roosters are present), there is no meaningful difference you’d notice when cooking or eating it. A 2015 study comparing the protein content of fertilized and unfertilized chicken egg yolks found changes in only 18 out of 225 identified proteins. The differences involved proteins related to blood vessel development and pathogen defense in the embryo, not anything that alters the egg’s taste, appearance, or nutritional value for the person eating it.
Federal regulations do draw a line, though. Eggs containing embryos that have developed to the blood ring stage or beyond (visible signs of a developing circulatory system) are classified as inedible and cannot legally be sold for human consumption. Eggs rejected from incubators are also classified as restricted and must be destroyed or processed for non-food uses.
Fertilized Eggs in Fertility Treatment
In the context of IVF (in vitro fertilization), “fertilized egg” refers to a human egg that has been combined with sperm in a laboratory. Embryologists examine these fertilized eggs closely at specific time points to assess their quality and likelihood of leading to a successful pregnancy.
The first assessment happens 16 to 18 hours after the egg is exposed to sperm. At this stage, specialists look for two small circular structures called pronuclei, one containing the mother’s chromosomes and one containing the father’s. The size, position, and internal patterns of these structures help predict how well the embryo will develop. An embryo where these structures are equal in size, centrally positioned, and show organized internal patterns is considered higher quality.
By day three, the embryo should have seven to ten cells. Embryologists evaluate whether the cells are roughly equal in size and how much fragmentation is present. Some fragmentation is normal in human embryos, but embryos with more than 25% fragmentation have significantly lower chances of successful implantation. The highest-graded embryos at this stage have eight evenly sized cells with minimal fragmentation.
By day five, the embryo reaches the blastocyst stage. This is now the most common point at which embryos are transferred to the uterus or frozen for later use, since reaching this stage is itself a sign of viability. Not all fertilized eggs make it this far. A significant portion stop dividing at earlier stages, which is a normal part of the process rather than an indication that something went wrong with the procedure.