The most reliable way to tell if an egg is fertilized is to crack it open and look at the germinal disc on the yolk, or to candle the egg with a bright light after a few days of incubation. Each method works best in different situations: cracking is definitive but destroys the egg, while candling lets you check without breaking the shell. The popular “float test” does not detect fertilization at all.
Check the Germinal Disc on a Cracked Egg
Every egg yolk has a small white spot on its surface called the germinal disc. This spot exists whether or not the egg was fertilized, but it looks noticeably different in each case. Learning to read this spot is the fastest and most accurate way to confirm fertilization if you don’t need to keep the egg intact.
In an unfertilized egg, the germinal disc (called a blastodisc) appears as a small, dense, irregularly shaped white dot. It’s solid-looking and compact, often with a slightly bumpy or uneven texture. Tiny bubble-like structures called vacuoles sometimes surround the center, giving it a rougher appearance.
In a fertilized egg, the disc (now called a blastoderm) looks like a bullseye or target. You’ll see a pale, nearly uniform whitish circle with a slightly denser white center. The key difference is a faint translucent ring surrounding that center. This ring forms because after fertilization, the embryonic cells divide rapidly. By the time the egg is laid, the disc already contains 50,000 to 60,000 cells arranged in a flat layer on the yolk’s surface. The translucent inner zone sits above a pocket of clear fluid deposited by the embryo, which is what creates that see-through “halo” effect. The outer ring stays opaque because it’s still attached directly to the yolk.
To see this clearly, crack the egg onto a white plate in good light. The germinal disc sits on top of the yolk. You may need to gently rotate the yolk so the disc faces upward. The bullseye pattern is subtle but distinct once you know what to look for.
Candle the Egg With a Bright Light
Candling means holding a bright, focused light against the eggshell in a dark room so you can see what’s happening inside without cracking it open. This is the standard method for anyone incubating eggs who wants to confirm development and remove duds. It won’t tell you much on the day the egg is laid, though. You need to wait for the embryo to grow enough to be visible.
You can use a dedicated egg candler, a bright LED flashlight, or even your phone’s flashlight pressed against the shell. Cup your hand around the egg and light source to block outside light, then look for the signs below.
Days 1 Through 7
The earliest visible sign is a small dark spot (the embryo) with thin red veins spreading outward from it, like a tiny spider web. You’ll also see a clear air cell at the wide end of the egg. An infertile egg, by contrast, looks uniformly yellow-orange inside with no veins and no dark spot. Most people can reliably distinguish fertile from infertile eggs by day 4 or 5 with white-shelled eggs.
Days 8 Through 14
The embryo is now large enough to see as a dark, moving shape. Blood vessels cover more of the shell’s interior, and you can often spot clear movement if you hold the egg still for a moment. The growing embryo takes up an increasingly large portion of the egg.
Days 15 Through 18
A large dark shadow fills most of the egg. The air cell at the top is noticeably bigger than before. You may see slight movement within the dark area, but the embryo is so large at this point that details are hard to make out.
Signs of a Dead Embryo
Not every fertilized egg develops successfully. A blood ring, which looks like a red circle inside the egg with no other growth, means the embryo died early. Broken or faded blood vessels, a dark ring with no movement, or an egg that looks dark but completely still late in incubation all indicate the embryo has stopped developing. These eggs should be removed from the incubator.
Brown Eggs Are Harder to Candle
Shell color makes a real difference. Brown eggs are more difficult to candle than white eggs because the pigment blocks more light. If you’re working with brown or dark-shelled breeds, wait until at least the fifth or sixth day before candling, since the embryo is simply too hard to see before then. Use the brightest light source you can find, and candle in a completely dark room for the best contrast.
The Float Test Doesn’t Work for This
A common claim online is that you can determine fertilization by placing an egg in water: if it sinks, it’s fertile, and if it floats, it’s not. This is a myth. The float test measures the size of the air cell inside the egg, which grows as moisture evaporates through the shell over time. A floating egg is simply an older egg with a larger air pocket. Storage conditions, humidity, and shell thickness all affect how quickly moisture escapes. The test tells you something about freshness but nothing about fertilization.
How to Store Fertile Eggs Before Incubation
If you’ve confirmed an egg is fertilized and want to incubate it, storage conditions matter. Fertile eggs should be collected soon after laying and kept at 50 to 65°F. They must not warm above 65°F unless you’re placing them directly into an incubator, because higher temperatures can trigger partial, uneven development that kills the embryo.
For chicken eggs, the recommended storage limit is one week, though viability can persist for up to three weeks with decreasing hatch rates. Turkey eggs can survive about four weeks in storage, while quail eggs lose viability after roughly two weeks. The sooner you incubate after collection, the better your chances of a successful hatch.
What Fertilization Actually Means
A fertilized egg is not the same thing as a developing egg. Fertilization happens inside the hen before the shell even forms. If a rooster has mated with the hen, sperm can remain viable in her reproductive tract for days, fertilizing multiple eggs over time. The first cell divisions begin while the egg is still inside the hen, and by the time it’s laid, the blastoderm already has tens of thousands of cells. But development pauses once the egg cools below body temperature. It won’t resume unless the egg is incubated at around 99 to 100°F. A fertilized egg sitting in your refrigerator will never develop further and is perfectly safe to eat. There is no nutritional or taste difference between fertilized and unfertilized eggs.