A fertilized chicken egg is an egg in which a hen’s egg cell has been joined by a rooster’s sperm, creating the potential for an embryo to develop into a chick. On the outside, it looks identical to the unfertilized eggs you buy at the grocery store. The difference is entirely internal, and unless the egg is incubated at the right temperature, that embryo never progresses beyond a tiny cluster of cells.
How Fertilization Happens
When a rooster mates with a hen, sperm travel into the hen’s reproductive tract and are stored in specialized structures called sperm storage tubules. Only about 1% of sperm make it into these tubules, where they can survive for a remarkably long time. A single mating can produce fertilized eggs for two to three weeks afterward, because the hen releases stored sperm each time a new yolk enters the upper part of her oviduct.
Fertilization happens before the egg white, membranes, and shell are added. The sperm meets the yolk shortly after it’s released from the ovary, and cell division begins almost immediately. By the time the hen lays the egg roughly 24 hours later, the fertilized cell has already divided into approximately 20,000 cells, forming a small disc of tissue called the blastoderm that sits on top of the yolk.
Fertilized vs. Unfertilized: What’s Different
Every chicken egg, fertilized or not, has a small white spot on the surface of the yolk called the germinal disc. This is the hen’s egg cell. In an unfertilized egg, this spot (technically called a blastodisc) stays as a simple, compact dot. In a fertilized egg, the germinal disc develops into a blastoderm, a multi-layered tissue five to six cells thick. To the naked eye, the difference is subtle: the fertilized spot tends to look slightly larger and may have a faint bullseye pattern with a lighter center, while the unfertilized spot appears more solid and uniform.
At the time of laying, the difference is genuinely hard to see without experience. If you crack a fresh egg into a dish, you’d be unlikely to tell whether it’s fertilized. The only reliable early method is candling, which involves holding a bright light behind the egg in a dark room. But even candling doesn’t reveal much until the egg has been incubated for several days.
What Happens During Incubation
A fertilized egg only develops into a chick if it’s kept at the right temperature. In a forced-air incubator, that means a steady 100°F (about 37.8°C) with 58 to 60% relative humidity. In a still-air incubator, the temperature needs to be slightly higher, around 102°F, to compensate for uneven heat distribution. The full incubation period for chickens is 21 days.
Development follows a predictable timeline. By day 7, candling reveals a small dark spot (the embryo) with red veins branching outward like a spider web, and you may see slight movement. By day 14, the developing chick takes up most of the egg’s interior, and very little light passes through. An infertile egg, by contrast, stays completely clear with no veins or dark spots at any point. Eggs should not be turned after day 18, and humidity is increased to 65% or higher during the final three days to help the chick hatch.
If a fertilized egg is never warmed to incubation temperature, the embryo’s development stays paused indefinitely. Refrigeration (below about 40°F) halts all cell activity. Even at hatchery storage temperatures of 60 to 65°F, the embryo remains dormant, though viability starts to decline if storage exceeds seven days. After 10 days of storage, embryo mortality rises noticeably and chicks that do hatch may show reduced growth.
Nutrition and Safety for Eating
The USDA states plainly: there is no nutritional difference between fertilized and unfertilized eggs. The protein, fat, cholesterol, and vitamin content are the same. The 20,000 cells in a freshly laid fertilized egg are microscopically small compared to the mass of the yolk and white, so they contribute nothing measurable to the egg’s nutritional profile.
Fertilized eggs are completely safe to eat. If they’ve been collected promptly and refrigerated (as standard practice on farms and in stores), no embryonic development has occurred. You would not be able to taste or see any difference. Most commercial egg operations don’t keep roosters with their hens, so virtually all grocery store eggs are unfertilized. Eggs from backyard flocks or farmers’ markets are more likely to be fertilized, but they’re equally safe and identical in flavor as long as they haven’t been sitting at warm temperatures for days.
Blood Spots Are Not a Sign of Fertilization
One of the most common misunderstandings is that a blood spot inside an egg means it was fertilized. It doesn’t. Blood spots happen when a small blood vessel ruptures during yolk release or while the egg is forming in the oviduct. The hen’s ovary and oviduct have an extensive blood supply, and occasionally a tiny hemorrhage gets incorporated into the egg. This can happen in both fertilized and unfertilized eggs. Blood spots are harmless and safe to eat, though many people prefer to remove them.
Why People Seek Out Fertilized Eggs
Some people buy fertilized eggs to hatch chicks at home, whether for backyard flocks or educational projects. Others purchase them based on a belief that fertilized eggs are more nutritious or “alive” with additional health benefits. While this belief is popular in certain food traditions, the nutritional data doesn’t support it. The practical reasons for choosing fertilized eggs come down to hatching, not eating. If you’re buying eggs purely for cooking, fertilization status makes no difference to what ends up on your plate.