The simplest way to tell if a lizard egg is alive is to hold a small light source against the shell in a dark room, a technique called candling. A living egg will show a network of reddish blood vessels or a visible embryo shadow, while a dead or infertile egg glows yellow with no internal structure. But candling isn’t the only method, and timing matters. Here’s how to assess your eggs at every stage.
Why You Should Never Rotate the Egg
Before you do anything else, understand this: turning a lizard egg can kill the embryo. Within the first day or two after laying, the developing embryo attaches to the inside of the shell. If the egg is flipped, the weight of the yolk can tear the delicate membranes connecting the embryo to its blood supply. This is why experienced breeders mark the top of each egg with a pencil or non-toxic marker the moment they find it, then always keep that side facing up.
If you need to pick up an egg for candling, lift it gently without rotating it and return it to the exact same position. A small dot or “X” on the top makes this easy to track.
Candling: The Most Reliable Check
Candling means shining a focused light through the eggshell to see what’s inside. A small LED flashlight or a phone light works well. Darken the room, cup the egg gently in your hand (top side still up), and press the light against the bottom or side of the shell.
What you see depends on how far along the egg is:
- First few days: A fertile egg often shows a small, dull orangey-yellow spot near the top. This is sometimes called the “Cheerio” because it looks like a small ring. It won’t be bright red yet. An infertile egg typically glows uniformly yellow with no distinct spot.
- One to two weeks: The yellowish spot gradually gains a reddish tint, and you may start seeing thin lines branching outward. Those lines are blood vessels, the clearest sign of a living embryo. Eggs incubated at warmer temperatures develop faster, so the exact timing varies by species and setup.
- Three weeks and beyond: A healthy egg becomes increasingly dark and opaque as the embryo grows. You’ll see less light passing through, with a large shadowy mass inside. A bright, clear egg at this stage with no shadow is almost certainly not viable.
Don’t panic if your first candling doesn’t show the classic red ring you’ve seen in online photos. Many new breeders expect a vivid red circle right away, but early-stage eggs look more like a blob of dull yellowish fluid with faint reddish light filtering through in one area. The dramatic veining photos you see online are usually taken at two to three weeks or later.
Visual Cues Without Candling
You can learn a lot just by looking at and gently touching the egg over time.
Healthy lizard eggs are white or off-white with a slight sheen. They feel firm but have a small amount of give, similar to soft leather in many gecko and anole species, or a harder, more calcified texture in species like bearded dragons. Over the incubation period, a living egg typically stays plump and may swell slightly as the embryo absorbs moisture from the substrate.
Dead or infertile eggs follow a different path. They often turn yellow, develop brown or greenish discoloration, or begin to collapse inward. A deflating egg that looks dented and doesn’t recover its shape after a day or two is likely no longer viable. Some dead eggs also develop a rubbery, translucent quality instead of staying opaque.
Mold: When to Worry and When to Ignore It
Finding fuzzy white growth on or near your eggs is alarming, but it doesn’t automatically mean the egg is dead. White fuzzy mold frequently grows on bits of organic debris sitting on the egg’s surface or the surrounding substrate. Experienced breeders report that this kind of surface mold has no effect on hatching, and they simply leave it alone.
The key distinction is where the mold originates. If the mold appears to radiate outward from a speck of debris near the egg, the egg is likely fine. Healthy eggs have a protective coating that functions almost like an immune system, preventing mold from penetrating the shell. But if the egg itself appears to be the source the mold is feeding on, with fuzzy growth spreading directly from the shell’s surface and eventually engulfing it, the egg has almost certainly gone bad. Infertile eggs and eggs where the embryo has died are the ones that get consumed by mold.
Smell is another useful signal. A healthy incubation container has a mild, earthy scent from the damp substrate. A noticeably “moldy” or sour smell, especially concentrated around one egg, suggests that egg is decomposing. If one egg goes bad, remove it promptly so the mold doesn’t spread to healthy eggs nearby.
Fertile vs. Infertile Eggs at Laying
Some eggs are infertile from the start, particularly if your lizard hasn’t been with a male (though females of many species lay eggs regardless of mating). Infertile eggs are often smaller, more irregularly shaped, or slightly yellow compared to fertile ones. In species that lay pairs, like leopard geckos, one egg may be fertile while the other is not.
Infertile eggs don’t develop the Cheerio-like spot when candled, even after a week. They stay uniformly yellow-orange inside, and within two to three weeks they typically start to discolor, shrink, or mold over. If you’re unsure, give the egg at least two weeks of proper incubation before deciding. Some eggs that look questionable early on surprise you with visible veins later.
What a Dying Egg Looks Like
Sometimes an egg starts developing normally and then the embryo dies partway through. When this happens, the blood vessels visible during candling stop spreading and may appear to darken or break apart. The egg might start losing its firmness, developing soft spots or subtle discoloration on the shell. A previously opaque egg that suddenly lets more light through when candled has likely lost its embryo.
Leaking fluid, a foul smell, or a dramatic color change to brown or green are late-stage signs. At that point, remove the egg to protect the rest of the clutch. If you’re incubating multiple eggs, check them at the same time so you can compare. A dead egg next to its healthy siblings is much easier to identify than one sitting alone.
How Often to Check
Resist the urge to candle every day. Each time you handle an egg, you risk jostling it or introducing temperature changes. Once a week is frequent enough for the first month. After that, every ten days to two weeks is plenty, since the embryo will be large enough that changes are obvious. Many breeders candle only two or three times across the entire incubation period and rely on external appearance the rest of the time.
Keep a simple log: date, egg appearance, candling results. Over a two-to-three month incubation (typical for many common pet lizard species), this record helps you spot trends. An egg that hasn’t changed at all over three consecutive checks, with no growth in shadow size and no new veining, is almost certainly not developing.